You want to check that your friend, Bill, has your accurate phone number, but you can't ask him directly. You have to write a question on a card which and give it to Ava. She will take the card to Bill and then return the answer to you.

What should you write on the card to ensure Bill can encode the message so that Ava cannot read your phone number?

Imagine you have eight balls, each of the same size.

One of the balls weights slightly more than the other seven.

Using a balance, find the heavier ball using only two weighings.

You are given a rectangular cake with a rectangular piece removed. The removed piece is of any size or orientation.

How would you cut the remainder of the cake into two equal halves with one straight cut of a knife?