Ireally dont get how to do this increase £42 by 10%

Hopefully this will help you:
100% of £42 is £42

to get 10% of 42 you divide £42 by 10 (since you divide 100% by 10 to get 10%)
that gives you £4.20

because it says increase by 10% you add this on to the normal price (£42)
£42+£4.20 = £46.20