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Your rich, eccentric uncle
just passed away,
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and you and your 99 nasty relatives have
been invited to the reading of his will.
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He wanted to leave
all of his money to you,
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but he knew that if he did,
your relatives would pester you forever.
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So he is banking on the fact
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that he taught you everything
you need to know about riddles.
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Your uncle left the following
note in his will:
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"I have created a puzzle.
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If all 100 of you answer it together,
you will share the money evenly.
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However, if you are the first to find
the pattern and solve the problem
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without going through all of the leg work,
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you will get the entire inheritance
all to yourself.
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Good luck."
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The lawyer takes you and your 99 relatives
to a secret room in the mansion
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that contains 100 lockers,
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each hiding a single word.
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He explains:
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Every relative is assigned a number
from 1 to 100.
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Heir 1 will open every locker.
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Heir 2 will then
close every second locker.
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Heir 3 will change the status
of every third locker,
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specifically if it's open,
she'll close it,
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but if it's closed, she'll open it.
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This pattern will continue until
all 100 of you have gone.
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The words in the lockers that remain
open at the end
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will help you crack the code for the safe.
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Before Cousin Thaddeus can even start
down the line,
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you step forward and tell the lawyer
you know which lockers will remain open.
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But how?
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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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The key is realizing that the number
of times a locker is touched
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is the same as the number of factors
in the locker number.
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For example, in locker #6,
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Person 1 will open it,
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Person 2 will close it,
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Person 3 will open it,
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and Person 6 will close it.
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The numbers 1, 2, 3, and 6
are the factors of 6.
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So when a locker has an even number
of factors
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it will remain closed,
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and when it has an odd number of factors,
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it will remain open.
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Most of the lockers
have an even number of factors,
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which makes sense because factors
naturally pair up.
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In fact, the only lockers that have
an odd number of factors
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are perfect squares
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because those have one factor that when
multiplied by itself equals the number.
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For Locker 9, 1 will open it,
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3 will close,
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and 9 will open it.
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3 x 3 = 9,
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but the 3 can only be counted once.
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Therefore, every locker that is
a perfect square will remain open.
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You know that these ten lockers
are the solution,
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so you open them immediately
and read the words inside:
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"The code is the first five lockers
touched only twice."
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You realize that the only lockers
touched twice have to be prime numbers
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since each only has two factors:
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1 and itself.
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So the code is 2,3,5,7,11.
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The lawyer brings you to the safe,
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and you claim your inheritance.
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Too bad your relatives were always
too busy being nasty to each other
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to pay attention
to your eccentric uncle's riddles.