SOLUTION: The sum of two numbers that differ by 3 is q. What is the smaller of the two numbers in terms of q?

x + y = q,   (1)  (where x is the bigger number, y is the smaller)
x - y = 3.   (2)

Add these two equations (both sides). You will get

2x = q + 3.

Hence, x = .

For y, distract (2) from (1). You will get

2y = q-3.

Hence, y = .

It is what you asked for.