57. Schools A, B, and C each contributed the same amount of money to buy desks and chairs. As a result, School A took 15 more sets of desks and chairs than School B, and School B took 15 fewer sets of desks and chairs than School C. Therefore, Schools A and C each paid 500 yuan to School B, so the cost of each set of desks and chairs is $\qquad$ yuan.
Reference answer: 100