A shopkeeper marks his goods 30% above his cost price but allows a discount of 10% at the time of sale. His gain is:

Explanation:-

Let the cost price be Rs. 100.
then the mark up price which is 30% above the cost price,
Mark price = (100 + 30% of 100) = Rs. 130
Shopkeeper gives a discount of 10% on mark up price, then the
Selling Price = (130 - 10% 0f 130) = Rs. 117.
Gain = 117-100 = Rs. 17
%gain = (17*100)/100 = 17%.

Short Cut method:

100(CP)==30%↑==>130(MP)==10%↓==>117(SP).
Gain = 17%.