question: There is space for 20 pencils in the box. If there are 4 pencils missing from the box, how many pairs of pencils are in the box?
answer: In the box there are 20 pencils - 4 pencils = <<20-4=16>>16 pencils.
  Dividing into pairs we have 16 pencils / 2 pencils/pair = 8 pairs of pencils
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