DAY 253
A father gave his son 10 cents for every math problem he got right, but the son had to pay his Dad 5 cents for each problem he got wrong. At the end of the school year, 120 problems had been done, and neither Dad nor son owed anything to the other. How many problems did the boy solve correctly?

Answer:

40...............Let X = the number of problems solved correctly........ 10 X - 5(120 - X) = 0.