A library charges a one-time membership fee of $20 and a fee of $3 per book borrowed. If David paid a total of $56 to the library, which of the following equations can be used to find , the number of books David borrowed?
$3b - 20 = 56
$20b + 3 = 56
$3b + 20 = 56
$20b - 3 = 56
