Answer:
Step-by-step explanation:
The monthly cost is C(t) = 6 + 0.05t, where t is the number of minutes talked.
Talking for 8 hours would cost C(t) = 6 + 0.05(8), or $6.40;
Talking for 20 hours would cost C(20) = 6 + 0.05(20), or $7.00.
The effective cost per minute in the first case is $6.40/8 min = $0.80/min, and that in the second case is $7.00/20 min = $0.35/min