Can you solve the penniless pilgrim riddle? - Daniel Finkel

6,359,559 views ใƒป 2018-06-04

TED-Ed


ืื ื ืœื—ืฅ ืคืขืžื™ื™ื ืขืœ ื”ื›ืชื•ื‘ื™ื•ืช ื‘ืื ื’ืœื™ืช ืœืžื˜ื” ื›ื“ื™ ืœื”ืคืขื™ืœ ืืช ื”ืกืจื˜ื•ืŸ.

ืชืจื’ื•ื: Ido Dekkers ืขืจื™ื›ื”: Allon Sasson
00:07
After months of travel, youโ€™ve arrived at Duonia,
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ืื—ืจื™ ื—ื•ื“ืฉื™ื ืฉืœ ืžืกืข, ื”ื’ืขืชื ืœื“ื•ืื•ื ื™ื”,
00:11
home to the famous temple thatโ€™s the destination of your pilgrimage.
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ื‘ื™ืชื• ืฉืœ ื”ืžืงื“ืฉ ื”ืžืคื•ืจืกื ืฉื”ื•ื ื™ืขื“ ืžืกืข ื”ืขืœื™ื™ื” ืœืจื’ืœ ืฉืœื›ื.
00:15
Entering from the northwest,
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ืืชื ื ื›ื ืกื™ื ืžืฆืคื•ืŸ ืžืขืจื‘,
00:17
you pass through the city gates and the welcome center,
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ื•ืขื•ื‘ืจื™ื ื“ืจืš ืฉืขืจื™ ื”ืขื™ืจ ื•ืžืจื›ื– ืงื‘ืœืช ื”ืคื ื™ื,
00:20
where youโ€™re given a map and a brochure.
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ืฉื ื ื•ืชื ื™ื ืœื›ื ืžืคื” ื•ื—ื•ื‘ืจืช.
00:23
The map reveals that the town consists of 16 blocks,
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ื”ืžืคื” ืžื’ืœื” ืฉื”ืขื™ืจ ืžื›ื™ืœื” 16 ื‘ืœื•ืงื™ื,
00:27
formed by five streets that run west to east,
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ืฉื ื•ืฆืจื™ื ืžื—ืžื™ืฉื” ืจื—ื•ื‘ื•ืช ืฉืขื•ื‘ืจื™ื ืžื–ืจื— ืžืขืจื‘,
00:30
intersecting five more that run north to south.
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ื•ื ื—ืชื›ื™ื ืขืœ ื™ื“ื™ ื—ืžื™ืฉื” ื ื•ืกืคื™ื ืฉืขื•ื‘ืจื™ื ืžืฆืคื•ืŸ ืœื“ืจื•ื.
00:33
Youโ€™re standing on the northernmost street facing east,
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ืืชื ืขื•ืžื“ื™ื ื‘ืจื—ื•ื‘ ื”ืฆืคื•ืŸ ืžืขืจื‘ื™ ืขื ืคื ื™ื›ื ืœืžื–ืจื—,
00:37
with the two blocks containing the gate and the welcome center behind you.
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ื›ืืฉืจ ืฉื ื™ ื”ื‘ืœื•ืงื™ื ืฉืžื›ื™ืœื™ื ืืช ื”ืฉืขืจ ื•ืžืจื›ื– ื”ืžื‘ืงืจื™ื ืžืื—ื•ืจื™ื›ื.
00:42
The templeโ€™s only entrance lies at the very southeast corner.
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ื”ื›ื ื™ืกื” ื”ื™ื—ื™ื“ื” ืœืžืงื“ืฉ ื ืžืฆืืช ื‘ืคื™ื ื” ื”ื“ืจื•ื ืžื–ืจื—ื™ืช.
00:46
Itโ€™s not a long walk, but thereโ€™s a problem.
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ื–ื• ืœื ื”ืœื™ื›ื” ืืจื•ื›ื”, ืื‘ืœ ื™ืฉ ื‘ืขื™ื”.
00:49
As you learn from the brochure,
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ื›ืคื™ ืฉืืชื ืžื’ืœื™ื ืžื”ื—ื•ื‘ืจืช,
00:51
Duonia imposes a unique tax on all visitors,
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ื“ื•ืื•ื ื™ื” ืžื˜ื™ืœื” ืžืก ื™ื™ื—ื•ื“ื™ ืขืœ ื›ืœ ื”ืžื‘ืงืจื™ื,
00:56
which must be paid when they arrive at their destination within the city.
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ืฉื—ื™ื™ื‘ ืœื”ื™ื•ืช ืžืฉื•ืœื ื›ืฉื”ื ืžื’ื™ืขื™ื ืœื™ืขื“ื ื‘ืชื•ืš ื”ืขื™ืจ.
01:00
The tax begins at zero,
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ื”ืžืก ืžืชื—ื™ืœ ื‘ 0,
01:02
increases by two silver for every block you walk east,
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ื•ืขื•ืœื” ื‘ืฉื ื™ ืžื˜ื‘ืขื•ืช ื›ืกืฃ ืขืœ ื›ืœ ื‘ืœื•ืง ืฉืืชื ื”ื•ืœื›ื™ื ืžื–ืจื—ื”,
01:06
and doubles for every block you walk south.
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ื•ืžื•ื›ืคืœ ืขืœ ื›ืœ ื‘ืœื•ืง ืฉืืชื ื”ื•ืœื›ื™ื ื“ืจื•ืžื”.
01:10
However, a recent reform to make the tax fairer
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ืขื ื–ืืช, ืจืคื•ืจืžื” ืขื“ื›ื ื™ืช ื›ื“ื™ ืœื”ืคื•ืš ืืช ื”ืžืก ืœื”ื•ื’ืŸ ื™ื•ืชืจ
01:13
halves your total bill for every block you walk north
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ืžื—ืœืงืช ืœืฉืชื™ื™ื ืืช ื”ืžืก ื”ืกื•ืคื™ ืขืœ ื›ืœ ื‘ืœื•ืง ืฉืืชื ื”ื•ืœื›ื™ื ืฆืคื•ื ื”
01:17
and subtracts two silver for every block you walk west.
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ื•ืžืคื—ื™ืชื” ืฉื ื™ ืžื˜ื‘ืขื•ืช ื›ืกืฃ ืขืœ ื›ืœ ื‘ืœื•ืง ืฉืืชื ื”ื•ืœื›ื™ื ืžืขืจื‘ื”.
01:21
Just passing through the gate and the welcome center
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ืคืฉื•ื˜ ืœืขื‘ื•ืจ ื“ืจืš ื”ืฉืขืจ ื•ืžืจื›ื– ื”ืžื‘ืงืจื™ื
01:24
means you already owe four silver.
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ืื•ืžืจ ืฉื›ื‘ืจ ืืชื ื—ื™ื™ื‘ื™ื ืืจื‘ืขื” ืžื˜ื‘ืขื•ืช ื›ืกืฃ.
01:27
As a pilgrim you carry no money and have no way of earning any.
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ื›ืขื•ืœื™ื ืœืจื’ืœ ืื™ืŸ ืœื›ื ื›ืกืฃ ื•ืื™ืŸ ืœื›ื ื“ืจืš ืœื”ืจื•ื•ื™ื— ืื•ืชื•.
01:31
Whatโ€™s more, the rules of your pilgrimage forbid you
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ื‘ื ื•ืกืฃ, ื”ื—ื•ืงื™ื ืฉืœ ื”ืขืœื™ื” ืœืจื’ืœ ืื•ืกืจื™ื ืขืœื™ื›ื
01:34
from walking over any stretch of ground more than once during your journeyโ€”
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ืœืœื›ืช ืขืœ ื—ืœืงืช ืื“ืžื” ื™ื•ืชืจ ืžืคืขื ืื—ื“ ื‘ืžื”ืœืš ื”ืžืกืข --
01:39
though you can cross your own path.
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ืื‘ืœ ืืชื ื™ื›ื•ืœื™ื ืœื—ืฆื•ืช ืืช ื”ืžืกืœื•ืœ ืฉืœื›ื.
01:42
Can you figure out a way to reach the temple without owing any tax
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ื”ืื ืืชื ื™ื›ื•ืœื™ื ืœืžืฆื•ื ื“ืจืš ืœื”ื’ื™ืข ืœืžืงื“ืฉ ื‘ืœื™ ืœื”ื™ื•ืช ื—ื™ื™ื‘ื™ื ื‘ืžืก
01:47
or walking the same block twice in any direction?
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ืื• ืœืœื›ืช ื‘ืื•ืชื” ื“ืจืš ืคืขืžื™ื™ื ื‘ื›ืœ ื›ื™ื•ื•ืŸ?
01:51
Pause here if you want to figure it out for yourself.
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ืขืฆืจื• ื›ืืŸ ืื ืืชื ืจื•ืฆื™ื ืœื’ืœื•ืช ื‘ืขืฆืžื›ื.
01:54
Answer in: 3
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ืชืฉื•ื‘ื” ืขื•ื“: 3.
01:55
Answer in: 2
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ืชืฉื•ื‘ื” ืขื•ื“: 2.
01:57
Answer in: 1
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ืชืฉื•ื‘ื” ืขื•ื“: 1.
01:58
You look at the map to consider your options.
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ืืชื ืžื‘ื™ื˜ื™ื ื‘ืžืคื” ื›ื“ื™ ืœืฉืงื•ืœ ืืช ื”ืืคืฉืจื•ื™ื•ืช.
02:01
Walking towards the temple always increases the tax,
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ื”ืœื™ื›ื” ืœื›ื™ื•ื•ืŸ ื”ืžืงื“ืฉ ืชืžื™ื“ ืชื’ื“ื™ืœ ืืช ื”ืžืก,
02:04
and walking away decreases it,
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ื•ื”ืœื™ื›ื” ื”ืจื—ืง ืžืžื ื• ืชืงื˜ื™ืŸ ืื•ืชื•,
02:06
so it seems like you can never reach it without owing silver.
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ืื– ื ืจืื” ื›ืื™ืœื• ืœืขื•ืœื ืœื ืชื•ื›ืœื• ืœื”ื’ื™ืข ืืœื™ื• ื‘ืœื™ ืœืฉืœื.
02:10
But what happens when you walk around a single block?
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ืื‘ืœ ืžื” ืงื•ืจื” ื›ืฉืืชื ื”ื•ืœื›ื™ื ืžืกื‘ื™ื‘ ืœื‘ืœื•ืง ืžืกื•ื™ื™ื?
02:14
If you start out owing four silver and go clockwise starting east,
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ืื ืืชื ืžืชื—ื™ืœื™ื ื›ืฉืืชื ื—ื™ื™ื‘ื™ื ืืจื‘ืขื” ืžื˜ื‘ืขื•ืช ื•ื”ื•ืœื›ื™ื ืขื ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ ื‘ื›ื™ื•ื•ืŸ ืžื–ืจื—,
02:18
your tax bill becomes six,
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ื”ืžืก ืฉืœื›ื ื™ื”ืคื•ืš ืœืฉืฉ,
02:20
then 12,
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ืื– 12,
02:21
then 10,
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ืื– 10,
02:23
then five.
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ื•ืื– ื—ืžืฉ.
02:24
If you looped again, youโ€™d owe seven,
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ืื ืชืœื›ื• ืžืกื‘ื™ื‘ ืฉื•ื‘, ืชื”ื™ื• ื—ื™ื™ื‘ื™ื ืฉื‘ืข,
02:27
14,
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14,
02:28
12,
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12,
02:29
and six.
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ื•ืฉืฉ.
02:30
It seems that each clockwise loop leaves you owing one extra silver.
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ื ืจืื” ืฉื›ืœ ืœื•ืœืื” ื‘ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ ืžืฉืื™ืจื” ืืชื›ื ื—ื™ื™ื‘ื™ื ืžื˜ื‘ืข ื›ืกืฃ ื ื•ืกืฃ.
02:35
What about a counterclockwise loop then?
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ืžื” ืขื ื ื’ื“ ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ?
02:38
Starting owing four again and going south first,
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ืืชื ืžืชื—ื™ืœื™ื ื—ื™ื™ื‘ื™ื ืืจื‘ืขื” ืฉื•ื‘, ื•ืื ื”ื•ืœื›ื™ื ื“ืจื•ืžื” ืงื•ื“ื,
02:41
your bill changes to eight,
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ื”ื—ืฉื‘ื•ืŸ ืฉืœื›ื ืขื•ืœื” ืœืฉืžื•ื ื”,
02:43
10,
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10,
02:44
five,
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02:44
and three.
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ื—ืžื™ืฉื”,
ื•ืฉืœื•ืฉื”.
02:46
Looping again youโ€™d owe six,
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ืกื™ื‘ื•ื‘ ื ื•ืกืฃ ื•ืชื”ื™ื• ื—ื™ื‘ื™ื ืฉื™ืฉื”,,
02:48
eight,
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02:48
four,
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ืฉืžื•ื ื”,
ืืจื‘ืขื”,
02:49
and two.
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ื•ืฉื ื™ื™ื.
02:50
Each counterclockwise loop actually earns you one silver.
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ื›ืœ ืกื™ื‘ื•ื‘ ื ื’ื“ ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ ืœืžืขืฉื” ืžืจื•ื•ื™ื— ืœื›ื ืžื˜ื‘ืข ื›ืกืฃ.
02:55
Thatโ€™s because any tax doubled,
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ื–ื” ื‘ื’ืœืœ ืฉื›ืœ ืžืก ืžื•ื›ืคืœ,
02:57
plus two,
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ื•ืขื•ื“ ืฉื ื™ื™ื,
02:58
halved,
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ืžื—ื•ืœืง ื‘ืฉืชื™ื™ื,
02:59
and minus two,
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ื•ืคื—ื•ืช ืฉื ื™ื™ื.
03:00
always ends up one smaller than it started.
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ืชืžื™ื“ ืžื’ื™ืข ืœืื—ื“ ืคื—ื•ืช ืžืžื” ืฉื”ืชื—ืœืชื.
03:03
The key here is that while the different taxes for opposite directions
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ื”ืžืคืชื— ื”ื•ื ืฉื‘ืขื•ื“ ื”ืžื™ืกื™ื ื”ืฉื•ื ื™ื ืœื›ื™ื•ื•ื ื™ื ืฉื•ื ื™ื
03:07
may seem to balance each other out,
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ืื•ืœื™ ื ืจืื™ื ืžืื•ื–ื ื™ื ืื—ื“ ืžื•ืœ ื”ืฉื ื™,
03:09
the order in which theyโ€™re applied makes a huge difference.
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ื”ืกื“ืจ ื‘ื”ื ื”ื ืžื•ืคืขืœื™ื ืขื•ืฉื” ื”ื‘ื“ืœ ื’ื“ื•ืœ.
03:13
You start off owing four silver,
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ืืชื ืžืชื—ื™ืœื™ื ืขื ืืจื‘ืขื” ืžื˜ื‘ืขื•ืช,
03:15
so four counterclockwise loops would get you down to zero.
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ืื– ืืจื‘ืขื” ืกื™ื‘ื•ื‘ื™ื ื ื’ื“ ื”ืฉืขื•ืŸ ื™ื‘ื™ืื• ืืชื›ื ืœืืคืก.
03:19
Unfortunately, itโ€™s not that simple, since you canโ€™t walk the same block twice.
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ืœืžืจื‘ื” ื”ืฆืขืจ, ื–ื” ืœื ืคืฉื•ื˜ ื›ืœ ื›ืš, ืžืื—ืจ ื•ืืชื ืœื ื™ื›ื•ืœื™ื ืœืœื›ืช ืžืกื‘ื™ื‘ ืœื‘ืœื•ืง ืคืขืžื™ื™ื.
03:24
But thereโ€™s another way to reduce your bill:
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ืื‘ืœ ื™ืฉ ื“ืจืš ื ื•ืกืคืช ืœื”ืคื—ื™ืช ืืช ื”ื—ืฉื‘ื•ืŸ:
03:27
walking one large counterclockwise loop through the city.
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ื”ืœื™ื›ืช ืกื™ื‘ื•ื‘ ืื—ื“ ื’ื“ื•ืœ ื ื’ื“ ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ ื‘ืขื™ืจ.
03:30
From your starting position, walk three blocks south.
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ืžื ืงื•ื“ืช ื”ื”ืชื—ืœื” ืฉืœื›ื, ืœื›ื• ืฉืœื•ืฉื” ืจื—ื•ื‘ื•ืช ื“ืจื•ืžื”.
03:34
You need to leave the southernmost street clear for the final stretch,
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ืืชื ืฆืจื™ื›ื™ื ืœื”ืฉืื™ืจ ืืช ื”ืจื—ื•ื‘ ื”ื›ื™ ื“ืจื•ืžื™ ืœื—ืœืง ื”ืื—ืจื•ืŸ,
03:37
so continuing counterclockwise means going east.
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ืื– ื”ืžืฉื™ื›ื• ื ื’ื“ ื›ื™ื•ื•ืŸ ื”ืฉืขื•ืŸ, ืžื” ืฉืื•ืžืจ ืžื–ืจื—ื”.
03:40
Walk two blocks to the eastern wall and you owe a whopping 36 silver.
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ืœื›ื• ืฉื ื™ ื‘ืœื•ืงื™ื ืœืงื™ืจ ื”ืžื–ืจื—ื™ ื•ืืชื ื—ื™ื™ื‘ื™ื 36 ืžื˜ื‘ืขื•ืช ื›ืกืฃ.
03:46
But now you can start reducing your bill.
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ืื‘ืœ ืขื›ืฉื™ื• ืืชื ื™ื›ื•ืœื™ื ืœื”ืชื—ื™ืœ ืœื”ืคื—ื™ืช ืืช ื”ื—ืฉื‘ื•ืŸ.
03:49
Three blocks north and one block west cuts it to 2.5.
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ืฉืœื•ืฉื” ื‘ืœื•ืงื™ื ืฆืคื•ื ื” ื•ื‘ืœื•ืง ืžืขืจื‘ื” ื—ื•ืชื›ื™ื ืืช ื–ื” ืœ 2.5.
03:53
You canโ€™t go west from here โ€”that would leave you with no way out.
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ืืชื ืœื ื™ื›ื•ืœื™ื ืœืœื›ืช ืžืขืจื‘ื” ืžืคื” - ื–ื” ืœื ื™ืฉืื™ืจ ืœื›ื ื“ืจืš ื”ื—ื•ืฆื”.
03:57
So you go one block south, and the remaining three blocks west,
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ืื– ืืชื ื”ื•ืœื›ื™ื ื‘ืœื•ืง ื“ืจื•ืžื”, ื•ืฉืืจ ืฉืœื•ืฉืช ื”ื‘ืœื•ืงื™ื ืžืขืจื‘ื”,
04:01
leaving you with a debt of -1 silver.
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ืžื” ืฉืžืฉืื™ืจ ืืชื›ื ื‘ื—ื•ื‘ ืฉืœ ืžื™ื ื•ืก ืžื˜ื‘ืข.
04:04
And since doubling a negative number still gives you a negative number,
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ื•ืžืื—ืจ ื•ื”ื›ืคืœืช ืžืกืคืจ ืฉืœื™ืœื™ ืขื“ื™ื™ืŸ ื ื•ืชื ืช ืžืกืคืจ ืฉืœื™ืœื™,
04:07
walking the three blocks to the south wall means the city owes you eight.
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ื”ืœื™ื›ื” ืฉืœื•ืฉื” ื‘ืœื•ืงื™ื ื“ืจื•ืžื” ืื•ืžืจ ืฉื”ืขื™ืจ ื—ื™ื™ื‘ืช ืœื›ื ืฉืžื•ื ื”.
04:13
Fortunately, thatโ€™s exactly enough
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ืœืžืจื‘ื” ื”ืžื–ืœ, ื–ื” ื‘ื“ื™ื•ืง ืžืกืคื™ืง
04:15
to get you through the final blocks to the temple.
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ื›ื“ื™ ืœืขื‘ื•ืจ ืืช ื”ื‘ืœื•ืงื™ื ื”ื ื•ืชืจื™ื ืœืžืงื“ืฉ.
04:18
As you enter, you realize what youโ€™ve learned from your pilgrimage:
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ื›ืฉืืชื ื ื›ื ืกื™ื, ืืชื ืžื‘ื™ื ื™ื ืžื” ื’ื™ืœื™ืชื ื‘ืขืœื™ื” ืœืจื’ืœ:
04:22
sometimes an indirect route is the best way to reach your destination.
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ืœืคืขืžื™ื ืžืกืœื•ืœ ืขืงื™ืฃ ื”ื•ื ื”ื“ืจืš ื”ื˜ื•ื‘ื” ื‘ื™ื•ืชืจ ืœื™ืขื“ ืฉืœื›ื.
ืขืœ ืืชืจ ื–ื”

ืืชืจ ื–ื” ื™ืฆื™ื’ ื‘ืคื ื™ื›ื ืกืจื˜ื•ื ื™ YouTube ื”ืžื•ืขื™ืœื™ื ืœืœื™ืžื•ื“ ืื ื’ืœื™ืช. ืชื•ื›ืœื• ืœืจืื•ืช ืฉื™ืขื•ืจื™ ืื ื’ืœื™ืช ื”ืžื•ืขื‘ืจื™ื ืขืœ ื™ื“ื™ ืžื•ืจื™ื ืžื”ืฉื•ืจื” ื”ืจืืฉื•ื ื” ืžืจื—ื‘ื™ ื”ืขื•ืœื. ืœื—ืฅ ืคืขืžื™ื™ื ืขืœ ื”ื›ืชื•ื‘ื™ื•ืช ื‘ืื ื’ืœื™ืช ื”ืžื•ืฆื’ื•ืช ื‘ื›ืœ ื“ืฃ ื•ื™ื“ืื• ื›ื“ื™ ืœื”ืคืขื™ืœ ืืช ื”ืกืจื˜ื•ืŸ ืžืฉื. ื”ื›ืชื•ื‘ื™ื•ืช ื’ื•ืœืœื•ืช ื‘ืกื ื›ืจื•ืŸ ืขื ื”ืคืขืœืช ื”ื•ื•ื™ื“ืื•. ืื ื™ืฉ ืœืš ื”ืขืจื•ืช ืื• ื‘ืงืฉื•ืช, ืื ื ืฆื•ืจ ืื™ืชื ื• ืงืฉืจ ื‘ืืžืฆืขื•ืช ื˜ื•ืคืก ื™ืฆื™ืจืช ืงืฉืจ ื–ื”.

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