Can you solve the wizard standoff riddle? - Dan Finkel

12,878,672 views ・ 2018-05-22

TED-Ed


Please double-click on the English subtitles below to play the video.

00:07
You’ve been chosen as a champion to represent your wizarding house
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in a deadly duel against two rival magic schools.
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Your opponents are fearsome.
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From the Newt-niz school,
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a powerful sorcerer wields a wand that can turn people into fish,
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but his spell only works 70% of the time.
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And from the Leib-ton school,
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an even more powerful enchantress wields a wand that turns people to statues,
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and it works 90% of the time.
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Lots are drawn, and you’re chosen to cast the first spell in the duel.
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The Newt-niz magician will go second,
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and the Leib-ton enchantress third,
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after which you’ll repeat casting in that order until only one of you is left.
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The rules of magic duels are strict,
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and anyone who casts out of order immediately forfeits the duel.
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01:00
Also, to prevent draws,
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the rules stipulate that if everyone’s still standing
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at the end of the first round,
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you’ll all be turned into cats.
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Now, you must choose a wand.
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Your wizarding house presents you with three options:
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the Bannekar, which binds one target with vines
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and casts effectively 60% of the time,
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the Gaussian, which turns one target into a tree
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and works 80% of the time,
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and the incredibly rare Noether 9000,
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which banishes one target to a distant mountaintop
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and casts perfectly 100% of the time.
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Your opponents are masters of strategy, as well as sorcery,
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and you know they’ll make the choices that maximize their own chances of success.
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Which wand should you choose
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and what strategy should you employ
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to have the greatest chance of winning the duel?
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Pause the video now if you want to figure it out for yourself!
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Answer in: 3
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Answer in: 2
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Answer in: 1
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You reach for the Noether 9000 first.
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After all, it makes sense to enter the duel with the most powerful wand.
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But before you pick it up, you consider what would happen.
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As the most dangerous wizard,
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you’d also be the target of the other two magicians,
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and you’d need to take care of the most dangerous of them first.
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But afterward, there’s a 70% chance you'd be struck down by the remaining wizard.
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That’s trouble.
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Maybe it’s better to take the Gaussian.
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It works 80% of the time,
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which means you wouldn’t be a target until the enchantress was incapacitated.
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But if you succeeded in transforming her,
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you’d probably be turned into a fish immediately after.
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If you transformed the sorcerer,
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the enchantress would almost certainly turn you to stone.
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It would really be better if you missed.
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And that’s when you have an idea:
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what if you took the Gaussian, then missed on purpose?
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Then, you would wait for the sorcerer to attack the enchantress,
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and you’d have an 80% chance of winning against the sorcerer.
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It’s a good idea, but there’s a problem;
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the sorcerer could also pass his turn
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and the enchantress, knowing that she couldn’t pass without becoming a cat,
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would cast her spell on one of you.
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And since you’re the most dangerous between you and the sorcerer,
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you’d be the target.
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And that’s when you see what you really need to do:
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take the weakest wand, the Bannekar, and miss on purpose.
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Now the sorcerer knows that he’ll be targeted by the enchantress
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and he’ll have to try to turn her into a fish to avoid being turned into stone.
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Seventy percent of the time he’d succeed
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and you’d have a 60% chance of winning the duel
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at the beginning of the next round.
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If he fails, chances are he’ll be turned to stone
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and you’d still have a 60% chance of winning the duel against the enchantress.
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There’s a slim 3% chance you’ll all be turned into cats,
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but when everything’s accounted for,
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you have better than even odds of winning with this strategy.
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And that’s the best you can do.
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Here’s what the probability of winning for the different strategies looks like.
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Who would’ve thought that the best way to take your shot
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would be to throw away your shot?
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