I think of a number, halve it, add on 9 and the answer is 22.5
I think of a number: call it x
halve it: that gives us x/2
add on 9:now we have x/2 + 9
and the answer is 22.5: so x/2 + 9 = 22.5
x/2 + 9 = 22.5
x/2 = 22.5 - 9
x/2 = 13.5
x = 2*13.5
x = 27