Answer: Cost per gb of data = y = $5.
Explanation:
Let the initial fee be 'x'.
Let the fees depending on the amount of data he use be 'y'.
After using 3 GB of data , he owes $30.
After using 5 GB of data , he owes $40.
According to question, our equations would be

So, Using elimination method , we get that

Hence, Cost per gb of data = y = $5.