Hi.
We need to write this as a mathematical expression.
Let's start by putting the part of the problem that refers to a "sum" in parentheses. We know one of the two numbers that are to be added together, but not the other. So we'll use "x" to stand in for that unknown number.
(-8 + x)
Next, we have to change the equation we're building by multiplying that sum by 5:
5(-8 + x)
Third, we have put into our equation the amount the whole thing equals:
5(-8 + x) = 15
Fourth, let's simplify on the left side:
5x - 40 = 15
Fifth, let's get the term with x in it all alone by adding 40 to both sides:
5x = 55
Sixth, let's get x itself all alone. To do this, we divide each side by 5:
x = 11
Finally, let's plug that value into our simplified equation to make sure it is correct:
5x - 40 = 15
5(11) - 40 = 15
55 - 40 = 15
Yes, it is the right answer.
