Two numbers have a sum of 50 If one number is p, express the other number in terms of p.

the other number is 50 - p

Consider a numeric example

2 numbers sum to 5, one of the numbers is 3 what is the other?

Obviously 2, that is 5 - 3 = 2 gives the number

In the same way the number here is p and 50 - p is the other number