question: Four books are arranged on a shelf. The first book is 31 mm thick while the second book is 50 mm thick. The third book is 5 mm less thick than the second book, and the fourth book is twice as thick as the first book. What is the total thickness of the four books?
answer: The third book is 50 - 5 = <<50-5=45>>45 mm thick.
  The fourth book is 31 x 2 = <<31*2=62>>62 mm thick.
  The total thickness of the four books is 31 + 50 + 45 + 62 = <<31+50+45+62=188>>188 mm.
  #### 188