You should know that:
10 10p coins make a pound.
100 1p coins make a pound.
Using coins, £4 is 4 pound coins.
So £4 can be written as 4 × £1.
Using coins, £4.20 is 4 pound coins and 2 10p coins.
£4.20 can be written as 4 × £1 + 2 × 10p
Using coins, £4.28 is 4 pound coins and 2 10p coins and 8 1p coins.
£4.28 can be written as 4 × £1 + 2 × 10p + 8 × 1p