A tradesman by means of a false balance defrauds to the extent of 8% in buying goods and also defrauds to 8% in selling. His gain percent is:

Since he make 8% profit so his purchase = 100 + 8 = 108
He make 8% more profit during sales so his sales = (108/100) × 108 = 116.64
so profit = 116.64 - 100
= 16.64
Short Cut (with increase decrease short cut)
X + Y + (xy/100)
= 8 + 8 + (8×8/100) ( since he his making profit so count is as a increase)
= 16.64