Three friends rented a car for a week and divided the cost equally. If a fourth friend had joined the three friends and the cost had been divided equally among the four friends, the cost to each of the original three would have been reduced by $15. What was the total cost of renting the car?

This was a data sufficiency on a GMAT review practice test. I figured what was sufficient to the problem but I wanted to see how to actually figure out the total cost. I tried setting it up but it seems I keep messing something up. Please help with the algebra setup for this. Thank you.

Eric

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