Edward C. answered 02/26/15
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Caltech Grad for math tutoring: Algebra through Calculus
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.