When you look at your choices, we see that 8 is not a factor of 36 and 9 is not a factor of 48. the last choice of 6 is not larger than 6, ...so 12 is the only appropriate answer as it is a factor of 36, 48, and larger than 6.
when will they be equal......so set them equal to each other 20 + 1.25d = 2.50d 20 = 2.50d - 1.25d 20 = 1.25d 20 / 1.25 = d 16 = d.....so they will be equal at 16 dvd rentals