12 copies of a book were sold for 1800 thereby gaining cost-

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12 copies of a book were sold for 1800 thereby gaining cost- price of 3 copies. The cost price of a copy is :

1. $ 120/-
2. $ 150/-
3. $ 1200/-
4. $ 1500/-

Correct Option: A

Let the cost price of 1 book be y.
∴ Cost price of 3 books = 3y
and, cost price of 12 books = 12y
Selling price of 12 books = 1800
⇒ 12y + 3y = 15y
⇒ 15y = 1800

∴ y =	1800	= 120
	15

Hence , The cost price of each book = $ 120