Mark has a voucher that gives him 22% off the prices at a hardware store. Estimate how much he will pay for an electric drill that normally costs £87.99. (non-calculator) (3)

It is important to remember this is an estimate, so we can simplify the numbers by saying they get 20% off and the drill costs £90. (1)

10% off would be £9, so 20% off is £18. (1)

They have 20% off so they pay £18 less. 90 - 18 = 72. 

Mark pays £72 for the drill. (1)