A school district is planning on buying new desks for its classrooms. Each secondary classroom is to have 10 more desks than an elementary classroom. For 15 elementary classrooms and 35 secondary classrooms, a total of 1200 desks are needed. The system of equations below represents this situation, where x represents the number of desks in an elementary classroom and y represents the number of desks in a secondary classroom. Which equation can be used to determine the number of desks in a elementary classroom, when solving by substitution?