Austin has 15 CDs, which is 3 less than his sister has. How many CDs does his sister have?

[math_celebrity]

Austin has 15 CDs, which is 3 less than his sister has. How many CDs does his sister have?

Let s be the number of CD's his sister has and a be the number Austin has

1. a = 15
2. a = s - 3

Substitute (1) into (2)
15 = s - 3

Add 3 to each side
s = 18