Mixture pricing — blend two teas and compute gain on selling price A tea producer blends two varieties: ₹18 per kg and ₹20 per kg in the ratio 5:3 by weight. If the blended tea is sold at ₹21 per kg, what is the gain percentage on cost?

Introduction / Context:
Mixture problems in profit-and-loss require finding the weighted-average cost price of the blend and then comparing it with the selling price to obtain the margin on cost.

Given Data / Assumptions:

• Component prices: ₹18/kg and ₹20/kg.
• Mixing ratio: 5 : 3 by weight.
• Selling price of blended tea: ₹21/kg.

Concept / Approach:
Average cost price = (5*18 + 3*20) / (5 + 3). Gain% = (SP − CP_avg) / CP_avg * 100.

Step-by-Step Solution:
Average CP = (90 + 60)/8 = 150/8 = ₹18.75 per kg.Profit per kg = 21 − 18.75 = ₹2.25.Gain% = 2.25 / 18.75 * 100 = 12%.

Verification / Alternative check:
If 8 kg are blended: total cost = 5*18 + 3*20 = ₹150; revenue at ₹21/kg for 8 kg = ₹168; profit = ₹18 which is 12% of ₹150.

Why Other Options Are Wrong:
8%, 10%, 15%, and 18% do not match the exact ratio-weighted average leading to ₹18.75 cost.

Common Pitfalls:
Using a simple (18+20)/2 average without ratio weighting, or computing profit% on SP instead of CP.

Final Answer:
12%