probably .5
i can on others if necessary.
well, because the $0.25 is an added cost depending on how many minutes, the answer would be y = 30 + 0.25x.
first 15 divide by 2 = 7.5
100 divided by 7.5 is 13.
15 / 2 = 7.5
100 / 7.5 = 13.