question: Each class in a school has 20 students. There are 3 classes. The school as a whole is 50% boys and 50% girls. The first class has 15 girls. The second class has 12 girls. How many boys are in the third class?
answer: The school has 60 students because 3 x 20 = <<3*20=60>>60
  The school has 30 girls because 60 x .5 = <<60*.5=30>>30
  The third class has 3 girls because 30 - 15 - 12 = <<30-15-12=3>>3
  The third class has 17 boys because 20 - 3 = <<20-3=17>>17
  #### 17