Question: Can you guide me through solving this problem: 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 = 16 pencils.
Dividing into pairs we have 16 pencils / 2 pencils/pair = 8 pairs of pencils