The plans will cost the same when the amount you have to pay for talking for "x" minutes on Plan A is the same has what you have to pay for talking for the same number of "x" minutes when using Plan B.
$$ Plan A = $$ Plan B
To find the charge on each plan we add the base rate to the per minute call rate for each.
Plan A = $27 + $0.11x
Plan B = $13 + $0.15x
Let's drop the $ sign for now and get rid of the decimal point by multiplying by 100.
2700 + 11x = 1300 + 15x
Subtracting 11x and 1300 from both sides:
4x = 1400
x = 350 min.
Using this result the plans both cost $65.50 for 350 min of talk time.
