I can tell you the number that you are thinking of!

Question

Pick and whole number, double it, add ten, divide by two. No subtract the original number from what you have now. I will be able to tell you that number!

Answer 1

In later math (that is, algebra) you will represent an unknown value using a letter, for example x.

Let x be the whole number that you selected.

"double it" -- that gives you 2*x

"add tem" -- that gives you 2*x + 10

"divide by two" -- now you have x + 5

"subtract the original number" - just 5 left

"I will be able to tell you that number" - Yes, the number is 5