Say to your friend, “Pick two numbers, both of them less than 10.”
Then say, “Choose either one of those numbers and multiply it by 5.”
Then, “Add 7.”
Then, “Multiply the result by 2.”
Then, “Add that number to the other number that you picked initially.”
Finally, you ask, “What’s your answer?”
Knowing that, you can now tell your friend the two numbers he originally chose, and also the one he started with and multiplied by 5.
Here’s how: From your friend’s final result, you subtract 14. That gives you a two-digit number made up of the two numbers your friend initially selected. The left-hand digit is the number he started with and the other number multiplied by 5. (ie divide the number by 5 to find out)