Answer:
a) 3.60
5.20
7.20
3.2 + .4M
b) at most, 42 minutes if only have $20
Explanation:
minutes = 1; 3.2 + (.4)(1) = 3.60
minutes = 5; 3.2 + .4(5) = 5.20
minutes = 10; 3.2 + .4(10) = 7.20
minutes = M; 3.2 + .4M
b) 3.2 + .4M ≤ 20
.4M ≤ 16.8
M ≤ 42
at most, 42 minutes if only have $20