Okay so the equation would be like this 30 + 0.10g = P
g = GB of data used
P = Total Price
Let's work this out now!
So. If his total bill was $42 lets subtract the 30 that comes out every month automatically, now we are just left with 12, if each GB costs 0.10 then all you would do to find out how many GB he used is do 12/0.10 (which is really just 12*10) which will get you 120