6th Grade Concentration Problems: Problem 4

Problem: Ben is buying black and red pens at the store. He buys a total of 66 pens. Red pens cost 5 dollars each, while black ones cost 9 dollars each. Because of the large amount he bought, the store owner was able to give him a discount, letting him to buy the total amount of red pens at 85% of their rightful price and buying the total amount of black pens at 80% of their rightful price. If Ben spent 18% less in all than the amount he was supposed to pay, how many red pens did he buy?

Key: Find ratio of red pens to black pens

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