28.9 I think is this, I hope this help
So, to figure out this problem you first need to solve for how much you have left after paying the base fee. Take the total ($84) and subtract by the base fee ($27). You will get 57 as your answer. Then, take $57 and divide it by $0.15 to get your answer. It is 380 minutes each month. The equation here is 84=27+0.15x.
