Let N = number of letters
Salvador wrote 20 letters each month for p months. So think about how many letters he wrote after 1 month (20), after 2 months (40), after 3 months (60), and so on. The way you figure out the number of letters is to multiply the number of months by 20, so the expression is
N = 20*p
You can check by plugging in values like 1, 2, or 3 for p that this expression is correct.