Answer
$6$ cents per minute
Work Step by Step
For a 10 minute call, the cost is $60$ cents.
For a 30 minute call, the cost was $180$ cents.
The increase in cost is:
$=180-60=120$ cents
The difference in time is:
$=30-10=20$ minutes
Thus, the average rate is:
$=\dfrac{120\text{ cents}}{20 \text{ minutes}}=6$ cents per minute