



 
Taha, Suppose the shopkeeper's cost for a pen is $C. To calculate the profit at 10% he multiplied $C by 0.10 to find the profit and then added it to $C to arrive at $13.20. So his calculation was
Solve this equation for $C and then recalculate the selling price, this time using 15% rather than 10%. I hope this helps,  


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