Question 389350
Let m = # minutes of call.  By assumption, {{{m >= 20}}}.  Then we have to solve the equation
0.99 + (m - 20)0.07 = 0.06m,
0.99 + 0.07m - 1.4 = 0.06m,
-0.41 = -0.01m,
m = 41, total # minutes after which it be less expensive to use AT&T