Phone company a charges a monthly fee of $42.50, and $0.02 for each minute talk time. phone company b charges a monthly fee of $25.00, and $0.09 for each minute of talk time.

We need to call for x minute
+ Phone Company A charges a monthly fee of $42.50, and $0.02 for each minute talk time. So we have to spend: $42.50+ $0.02x
+ Phone company B charges a monthly fee of $25.00, and $0.09 for each minute of talk time. So we have to spend: $25.00+ $0.09x

We solve for x: $42.50+ $0.02x> $25.00+ $0.09x
or $42.50- $25.00 > $0.09x- $0.02x
and we have $0.07x<$27.50
or x< 27.50:0.07 and x< 393.86

The answer is:
If we have to call much time, at least 394 minutes, we should choose A
If not, choose B

Wow. that is a really cool question.