In a laundromat, washing machine a was used 48 times in a month. this was 4 times the number of times washing machine b was used in that same month. which set of equations shows the number of times washing machine b was used during that month?
a. ? + 4 = 48 , 48 - 4 = ?
b. ? + 4 = 48 , 44 + 4 = ?
c. ? x 4 = 48 , 12 x 4 = ?
d. ? x 4 = 48 , 12 + 4 = ?