we are told that a plan costs $25 per month for 10GB and each extra gigabyte costs $10. If C is the amount she has to pay and "x" the number of additional Gigabytes during the month, then we have the following relationship:

If during the month 3 extra Gb were consumed, that means that x = 3, and the total cost to pay is:

Solving the operations:

Therefore, the cost to pay is $55.