Make x the subject
yx + q = r

The crucial point in this problem is the amount of the data usage. Since it's not given by reasoning with math logic we can find the correct answer. Let's find that if we spend 5$ in the first case, how much amount of data we would be able to use. By setting

0.002$1 KB

5$x and x=2500 KB or 2.5 MB

From this fact, we can yield that if you are willing to use more than 2.5 MB of data, it is mathematically correct to choose the second option since in this case you will save money.