Name: 【TED-Ed】Can you solve the locker riddle? - Lisa Winer
Uploaded: 2016-04-03T02:36:48.000Z
Duration: 3 min 50 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Adverbs of degree

Your rich, eccentric uncle  just passed away,

and you and your 99 nasty relatives have been invited to the reading of his will.

He wanted to leave  all of his money to you,

but he knew that if he did, your relatives would pester you forever.

that he taught you everything  you need to know about riddles.

Your uncle left the following  note in his will:

If all 100 of you answer it together, you will share the money evenly.

However, if you are the first to find the pattern and solve the problem

without going through all of the leg work,

you will get the entire inheritance  all to yourself.

The lawyer takes you and your 99 relatives to a secret room in the mansion

Every relative is assigned a number from 1 to 100.

Heir 2 will then  close every second locker.

Heir 3 will change the status of every third locker,

specifically if it's open,  she'll close it,

This pattern will continue until all 100 of you have gone.

The words in the lockers that remain open at the end

will help you crack the code for the safe.

Before cousin Thaddeus can even start down the line,

you step forward and tell the lawyer you know which lockers will remain open.

Pause the video now if you want to figure it out for yourself!

The key is realizing that the number of times a locker is touched

is the same as the number of factors in the locker number.

The numbers 1, 2, 3, and 6  are the factors of 6.

So when a locker has an even number of factors

and when it has an odd number of factors,

Most of the lockers  have an even number of factors,

which makes sense because factors naturally pair up.

In fact, the only lockers that have an odd number of factors

because those have one factor that when multiplied by itself equals the number.

Therefore, every locker that is  a perfect square will remain open.

You know that these ten lockers  are the solution,

so you open them immediately and read the words inside:

"The code is the first five lockers touched only twice."

You realize that the only lockers  touched twice have to be prime numbers

Too bad your relatives were always too busy being nasty to each other

to pay attention  to your eccentric uncle's riddles.

Subtitles ListPlay Video

【TED-Ed】Can you solve the locker riddle? - Lisa Winer

pattern

person

nasty

count