Subhash buys a tape recorder at 9/10 of its listed selling price and sells it at 8% above the listed selling price. Determine his gain percentage on cost.

Introduction:
When purchase and sale are expressed relative to a common reference (the listed selling price), it is convenient to treat that reference as a unit, compute the actual cost and selling price, and then form the gain percentage on cost.

Given Data / Assumptions:

• Let listed price be L.
• Cost price = 9/10 of L.
• Selling price = 1.08L (8% above L).

Concept / Approach:
Express both CP and SP in terms of L, then compute gain = SP - CP and divide by CP to get the percentage gain on cost.

Step-by-Step Solution:
CP = 0.9LSP = 1.08LGain = 1.08L - 0.9L = 0.18LGain% = 0.18L / 0.9L * 100 = 20%

Verification / Alternative check:
Let L = Rs 1000. Then CP = Rs 900, SP = Rs 1080, gain = Rs 180, which is 20% of Rs 900. The percentage is unchanged for any L since both prices scale with L.

Why Other Options Are Wrong:

• 9% and 10%: arise from mixing the base (using SP instead of CP).
• 180%: a misread of the rupee gain 180 in the numeric example.

Common Pitfalls:

• Computing percentage on SP rather than on CP.
• Confusing 8% of L with 8% of CP.

Final Answer:
20%