question: If Mark weighs 150 pounds and Susan weighs 20 pounds less than Mark.  And their friend Bob weighs twice as much as Susan.  What is the average weight of the 3 friends?
answer: Susan weighs 150 pounds - 20 pounds = <<150-20=130>>130 pounds.
  Bob weighs 2 * 130 pounds = <<2*130=260>>260 pounds.
  The friends total weight is 150 + 130 + 260 pounds = <<150+130+260=540>>540 pounds.
  The friends' average weight is 540 pounds / 3 = <<540/3=180>>180 pounds.
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