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