If 9 is added to 1/3 of a number, the result is 15. What is the number?

If 9 is added to 1/3 of a number, the result is 15. What is the number?

The phrase a number means an arbitrary variable, let's call it x:
x

1/3 of a number means we multiply x by 1/3:
x/3

9 is added to 1/3 of a number:
x/3 + 9

The phrase the result is means an equation. so we set x/3 + 9 equal to 15
x/3 + 9 = 15

To solve this equation for x, we type it in our search engine and we get:
x = 18