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

Answer: Option D

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% of 130) = Rs. 117
Gain = 117 - 100 = Rs. 17
$$\%\text{gain}=\frac{17\times 100}{100}=17\%$$

Short Cut method:
100(CP) == 30%↑ ⇒ 130(MP) == 10%↓ ⇒ 117(SP)
Gain = 17%