A 30cm long piece of string is cut into two pieces. One piece is twice as long as the other. How long is the longer piece?
Solution:
If the long piece is twice as long as the short piece, then we can imagine cutting the original piece of string into three pieces and then joining two of them together to make the long piece.
Each of the three pieces would be one third of 30cm, or 10cm long, so the longer piece must be 20cm long.