A shop-keeper purchases $$11$$ knives in Rs. $$10$$ and sells them at the rate of $$10$$ knives for Rs.$$11$$. He earns a profit of
Correct option is A. $$21$$ %
Rate of cost price of knives = Rs 10 for 11 knives.
So, Cost Price (CP) for a knives $$=\dfrac{10}{11}\ Rs$$
Rate of selling price of knives = Rs 11 for 10 knives.
So, Selling Price (SP) for a pen $$=\dfrac{11}{10}\ Rs$$
$$\therefore Profit = SP-CP=\dfrac{11}{10}-\dfrac{10}{11}=\dfrac{121-100}{110}=\dfrac{21}{110}\ Rs$$
$$\Rightarrow Profit\ percentage = \dfrac{Profit\times 100}{CP}=\dfrac{\dfrac{21}{110}\times 100}{\dfrac{10}{11}}=21\%$$