First, we want to find the difference in monthly fee and per minute costs.
20 - 15 = 5
0.25 - 0.05 = 0.20
So basically we now know that we need to make up for 5 dollars of monthly cost difference, and for every minute we talk with the two plans at the same time, it's different by 0.20.
Now, we just can divide 5 dollars by the difference of 0.20, which you would get 25 minutes from.
So you would have to talk for 25 minutes for the two plans to be the same cost.
Assuming that you used them at the same time, not just used B or A.
