#### Answer

(a) The actual cost for a 90 minute call is:
Penny Saver \$0.84
Most Minutes \$0.715
USA Card \$1.15
(b) Use the actual cost from (a) to find the unit cost.
Penny Saver \$0.009/min
Most Minutes \$0.008/min
USA Card \$0.013/min
The best buy is Most Minutes.

#### Work Step by Step

(a) To find the actual total cost, add the connection fee to the per minute charge for 90 minutes.
Penny Saver $0.39 + (90)(0.005)$ = $0.39+0.45$ = 0.84
Most Minutes $0.49 + (90)(0.0025)$ = $0.49+0.225$ = 0.715
USA Card $0.25 + (90)(0.01)$ = $0.25+0.90$ = 1.15
(b) Use the actual cost from (a) to find the unit cost.
Penny Saver $\frac{0.84}{90 min}$ = $\frac{0.84\div90}{90\div90 min}$ = $\frac{0.009}{1 min}$ = 0.009/min
Most Minutes $\frac{0.715}{90 min}$ = $\frac{0.715\div90}{90\div90 min}$ = $\frac{0.008}{1 min}$ = 0.008/min
USA Card $\frac{1.15}{90 min}$ = $\frac{1.15\div90}{90\div90 min}$ = $\frac{0.013}{1 min}$ = 0.013/min