Alloy cost from metals A and B:\nA costs Rs 2000/kg, B costs Rs 400/kg, mixed in 3:1 (A:B). Find the cost of 8 kg of alloy.

Introduction / Context:
Weighted average cost per kg = (3 parts of A at 2000 + 1 part of B at 400)/4 parts.

Given Data / Assumptions:
Mix ratio A:B = 3:1.

Concept / Approach:
Cost per kg = (3*2000 + 1*400)/4 = (6000 + 400)/4 = 1600 Rs/kg.

Step-by-Step Solution:

Note:
The correct total is Rs 12800. Options show 6400 also; ensure 8 kg is recognized. Since the computed total is Rs 12800, we select 12800.

Why Other Options Are Wrong:
6400 equals 4 kg at Rs 1600/kg; 9800 not aligned; 1600 is per-kg, not total.

Common Pitfalls:
Stopping at per-kg cost or multiplying wrongly by 4 instead of 8.

Final Answer:
12800