Two numbers have a sum of 20. If one number is p, express the other in terms of p.

Two numbers have a sum of 20. If one number is p, express the other in terms of p.

If the sum is 20 and one number is p, then let the other number be q.
We have: p + q = 20

We want q, so we subtract p from each side:
q = 20 - p