Calculus Optimization Problem

I know i need two equations and i end up finding the derivative, but i am really struggling with this problem

The owners of a potato processing plant permit nearby farmers to rent their machines during the weekends. The rental fee is $50 per machine plus $30 per hour to rent the plant regardless of the number of machines used. How many machines should a farmer rent to minimize his cost for processing 25,000 bushels of potatoes if each machine can produce 75 bushels per hour?