A = 40 + .1(M-500) for M>500
B = 25 + .05M
20 minutes a day for 30 days = 600 minutes per month
A(600) = 40+.1(100) = $50
B(600) = 25 + .05(100) = $30
Plan B is less expensive
40+.1(M-500)= 25+.05M
40-25- .1(500) = .05-.1M
.05M = 35
M= 35/.05 = 700 minutes will have the same cost under plan A or B
40+.1(200) = $60 under plan A
25+.05(700) = $60 under plan B