Math Homework Help

You write down the following 8 digit number on a piece of paper
1 2 3 4 5 6 7 9
Then ask your friend to circle one of the digits. Say 5

You then ask your friend to multiply the 8 digit number by 45, and magically the result ends up being:

1 2 3 4 5 6 7 9
x 4 5
—————
5 5 5 5 5 5 5 5

with the answer as a raw of the chosen number 5

The Secret!!!

When your friend circles a number, you need to multiply the chosen number by 9 in your head. Then you need to ask your friend to multiply the 8 digit number by the number you have worked out and you magically get the answer as a raw of the chosen number.