A man bought some rice at the rate of ? 15/kg and some other quantity at the rate of ? 24/kg. He mixed them and sold at the rate of ? 21.60/kg gaining a profit of 20%. What was the respective ratio of the two varieties ?

The correct answer is (b), 2:1.

Let $x$ be the quantity of rice bought at 15/kg and $y$ be the quantity of rice bought at 24/kg. We know that the total quantity of rice is $x+y$. We also know that the total cost of the rice is 15x+24y, and the total selling price is 21.6(x+y). We can set up an equation to find the profit:

15x+24y = 21.6(x+y)

15x+24y = 21.6x+21.6y

3x = 0.6y

x = 0.2y

This means that the ratio of the two quantities of rice is 2:1.