Bill and Bernie are playing a game to find out which coins (1p, 2p, 5p, 10p and 20p) roll down a hill. They discover that a 20p coin doesn't roll very well, as it isn't round like the others. They then add up the total amount of money that rolled (1p + 2p +3p + 5p + 10p), showing the answer at each step in the calculation. The difference between the amount which rolled down hill and that which did not is then calculated: 20p -18p = 2p.
Children could have fun with a 'coin sport' activity. They could have a collection of coins with a total value of less than 20p. They could then stand at a distance and throw coins into a bucket. They could then add the coins together and subtract the total from 20p. As they improve, the total value of coins could be increased and children could subtract from 30p.