**SOD**IS A HEBREW WORD FOR SECRET!

A dialog between two mathematicians:

Ted: I have two numbers x and y, where x + y = z. The sum of the digits of x is 43 and the sum of the digits of y is 68.

Can you tell me the sum of the digits of z?

Fred: I need more information. When you added x and y how many times did you have to carry?

Ted: Let’s see. … It was five times.

Fred: Then the sum of the digits of z is ** 66**.

Ted: That’s right! How did you know?

**How did he know? **

Credit: USC mathematician Solomon W. Golomb