Assume that the number of the minutes of calls is x minutes
For the first plan:
There is a monthly amount of $15
Additional cost $0.11 for each minute
Then the cost of the calls is

For the second plan:
There is a monthly amount of $8
Additional cost $0.16 for each minute
Then the cost of the calls is

If the two plans cost the same, then equate the two expressions above

Now let us solve the equation to find x
Subtract 8 from both sides

Subtract 0.11x from both sides

Divide both sides by 0.05

For 140 minutes the two plans have equal cost