A stationery shop owner buys three types of notebooks. The first type costs him 5 notebooks for ₹20, the second 2 notebooks for ₹9, and the third 10 notebooks for ₹45. He mixes them in the ratio 3 : 4 : 3. If he sells all the notebooks at 4 for ₹19, what is his approximate gain or loss percentage?

Correct — B.
Cost per notebook: 20 ÷ 5 = ₹4, 9 ÷ 2 = ₹4.50, 45 ÷ 10 = ₹4.50
Take one lot of 3 + 4 + 3 = 10 notebooks
Cost: 3 × 4 + 4 × 4.50 + 3 × 4.50 = 12 + 18 + 13.50 = ₹43.50

Sale at 4 for ₹19: 10 × 19 ÷ 4 = ₹47.50
Gain: 47.50 − 43.50 = ₹4
Gain %: 4 ÷ 43.50 × 100 = 9.195…% ≈ 9.2% profit → option (b)

Why the others are wrong

• (a)
  Loss of 9.2% — Loss is the wrong direction. The 10 notebooks cost ₹43.50 and sell for ₹47.50, so he gains ₹4. The 9.2% is right, the word is not.
• (c)
  Profit of 10% — 10% profit on a ₹43.50 cost would need sales of ₹47.85. The notebooks bring in ₹47.50, a gain of ₹4, which is 9.2% of cost.
• (d)
  Loss of 10% — Loss of 10% fails twice: it would mean sales of ₹39.15, but the 10 notebooks sell for ₹47.50, above their ₹43.50 cost.

Reduce every 'n for ₹x' price to a price per notebook first: ₹4, ₹4.50 and ₹4.50.

Then cost one complete lot in the mixing ratio. 3 + 4 + 3 = 10 notebooks cost ₹43.50, an average of ₹4.35 each. Selling at 4 for ₹19 is ₹4.75 each, so every notebook earns 40 paise on a cost of ₹4.35.

Key facts

• 'n for ₹x' means a unit price of x ÷ n: 5 for ₹20 is ₹4 each.
• Profit % is taken on cost: (sale − cost) ÷ cost × 100.
• Costing one lot of exactly the ratio total, here 3 + 4 + 3 = 10 notebooks, avoids fractions of a notebook.

Common traps

• Averaging the three unit costs equally, (4 + 4.50 + 4.50) ÷ 3 = ₹4.33, which ignores the 3 : 4 : 3 ratio.
• Taking the gain on the selling price: 4 ÷ 47.50 ≈ 8.4%.

No directly related past PYQ was found.