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A trader bought four varieties of coffee whose weights are in the ratio $1:2:3:4$. Their respective prices per kg are Rs. 100, Rs. 120, Rs. 150, and Rs. 180. What is the average cost per kg of the mixture?

1. Rs. 145
2. Rs. 158
3. Rs. 155
4. Rs. 151

Correct answer: Rs. 151

Solution

The average cost per kg of a mixture is the weighted average of the component prices. Using weights $1,2,3,4$, total cost $=100\cdot1+120\cdot2+150\cdot3+180\cdot4=1510$ and total weight $=1+2+3+4=10$. So the average cost per kg is $1510/10=Rs.\ 151$.

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