In a school fund-raiser, 10 students in class A sold an average (arithmetic mean) of 4 boxes of cookies. In class B, 15 students sold an average of 9 boxes. What was the average number of boxes of cookies sold by the 25 students?
  1. 13/25

  2.   1

  3.   6 1/2

  4.   7

  5. 13 6/13