On selling a pen at 5% loss and a book at 15% gain, Karim gains Rs. 7. If he sells the pen at 5% gain and the book at 10% gain, he gains Rs. 13. What is the cost price of the book in Rupees?
Assuming the cost price of pen = 100p and the cost price of book = 100b
So, on selling a pen at 5% loss and a book at 15% gain, net gain = -5p+15b = 7 ....1
On selling the pen at 5% gain and the book at 10% gain, net gain = 5p+10b = 13 .....2
Adding 1 and 2 we get, 25b=20
Hence 100b= 20*4=80,
C is the answer.
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