A store sells pencils in small and large boxes. A large box has ten times as many pencils as a small box. Mr. Adachi buys 5 small boxes and 2 large boxes, for a total of 250 pencils. Mr. Adachi sets up a system of equations to model this situation. One of the equations is y = 10x. What is the other equation? Enter your answer in ax + by = c form, where a and b are whole numbers with no common factor other than 1.