Given:
Runfast = 25 + 10g
BA&D = 18 + 15g
Runfast = 35 per month
35 = 25 + 10g
35 - 25 = 10g
10 = 10g
10/10 = g
1 = g
BA&D = 33 per month
33 = 18 + 15g
33 - 18 = 15g
15 = 15g
15/15 = g
1 = g
If g = 2;
Runfast = 25 + 10(2) = 25 + 20 = 45
BA&D = 18 + 15(2) = 18 + 30 = 48
Based on my computation, if the gig of data is only equivalent to 1g then BA&D is cheaper. However, if 2gig or above is used, then Runfast is cheaper.