Question

The sum of a number and 8 is 15

Answer 1

You mean, what is this number?

we can rephrase it like this: the sum of a number x and 8 is 15

which is:

x+8=15

we subtract 8 from both sides and we get
x=7

so the answer is 7.

Answer 2

Let the missing number be known as 's'

s + 8 = 15

Subtract 8 from both sides of the equation.

s + 8 = 15
     -8    -8
---------------------
s = 7.