Nasim has a smart phone data plan that costs $25 per month that includes 9 GB of data, but will charge an extra $20 per GB over the included amount. How much
would Nasim have to pay in a month where he used 5 GB over the limit? How much
would Nasim have to pay in a month where he used went over by x GB?

Answer

Both are correct.

Explanation
The both equations are the same.
When we say that y=x+10, we can make x the subject of the formula and get;
y=x+10
y-10=x
x=y-10.

If the substitution is done correctly, that is x+10 for y and y-10 for x in the other equation then, both would be correct.