A dishonest dealer claims to sell his goods at cost price but uses a weight of $950$ g in place of $1$ kg. What is his gain percent?

**Principle.** When the dealer cheats on the weight while selling at CP, $\text{Gain}\% = \dfrac{\text{Error}}{\text{True Value} - \text{Error}} \times 100$. **Apply.** Error $= 1000 - 950 = 50$ g, true value $= 1000$ g. Gain $\% = \dfrac{50}{950} \times 100 = \dfrac{100}{19} \approx 5\tfrac{5}{19}\%$. **Why option A is wrong.** $5\%$ would be the answer if the gain were on the *true weight* (50/1000). The gain is on *what he actually gives*, which is $950$ g — the denominator is true − error, not true.