first find out what 10% is
then half the answer
then add both the 10% and the 5% up to retrieve the anwer.
then take this away from your original answer
e.g. 15% 0ff 25.99
10% is 2.599 or 2.60 to 1d.p
5% is half this which is 1.2995 or 1.30 to 1d.p
so 2.60 + 1.30 = 3.90
therefore 25.99 -3.90 = 22.09 to 1 d.p