i would calculate this by taking 10% which is 1.5 (that's easy to work out because you just move the decimal point when place).
then another 5% (to give 15%) is half as much again = 0.75
1.5 + 0.75 = 2.25
15 - 2.25 = 12.75
If you're doing this on a calculator then it's 0.85 x 15
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