Question 98128

the cost for talking for time t = 0.10 + (t-1).05

since cost should not exceed $20

0.10 + (t-1).05 <=20
10 + (t-1)5 <=2000  multiplying by 100 throughout

2 + t-1 <=400 dividing by 5 
t+1<=400
t<=399

maximum time a person could talk is 399 minutes