If 10 pens cost 60 rupees, what is the cost of 15 such pens?
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Solution
Let us suppose the cost of 15 pens is x rupees.
The number of pens and their cost vary in direct proportion. Let us express the ratios and obtain an equation.
∴ 10x = 900 ∴ x = 90 (Dividing both side by 90)
Cost of 15 pens is 90.