Question: There are five phones on a phone plan. The main phone costs twice as much as each additional phone. If the main phone plan costs $20, how much does the whole phone plan cost?
Solve this problem. 
Answer: 
Each additional phone costs 20 / 2 = $10.
Thus, the four additional phones cost 4 * 10 = $40 in all.
Thus, the whole phone plan cost $40 + $20 = $60.