What does a kg of the merchant's blend cost him?

Explanation

1. 01

   Cost: $30 \times 360 + 10 \times 480 = 10{,}800 + 4{,}800 = 15{,}600$ rupees for 40 kg.

2. 02

   $\frac{15{,}600}{40} = 390$ rupees a kg.