Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$12$ cents per minute
For a 5 minute call, the cost is $60$ cents. For a 10 minute call, the cost was $120$ cents. The increase in cost is: $=120-60=60$ cents The difference in time is: $=10-5=5$ minutes Thus, the average rate is: $=\dfrac{60\text{ cents}}{5 \text{ minutes}}=12$ cents per minute