Answer
$12$ cents per minute
Work Step by Step
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