The Pen percentage problem is as follows :
Suppose I go to a bookstall and purchase 11 pens for 10 Rs. But then I go ahead and sell all those pens at the rate of : 10 pens for 11 Rs. What is my profit percentage ? 
a). 80 %
b). 21 %
c). 12 %
d). 10 %
The solution is given as follows :
EXPLANATION :
Consider how much I spent Initially : 10 rs for 11 pens.
Now I sold all those pens at a new price. So the first 10 pens sold for 11 Rs. and the remaining one pen sold for 1.1 Rs (Since 10 sold for 11 Rs)
Hence I now have 11 + 1.1 = 12.1 Rs.
Since I initially spent 10 Rs and I now have 12.1 Rs, my profit percentage will be ((12.1 – 10)/10) * 100% = 21%
Hence the answer is (b) 21 %
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