12 copies of a book were sold for Rs. 1800, thereby gaining the cost price of 3 copies. The cost price of a copy is-
Let the cost of the 12 books be 12x
Selling price is 1800
So, profit = 1800 - 12x
But 1800 - 12 x = 3x
So, x = 1800/15 = 120Â
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