A fruit vendor sells apples at cost price but uses a false weight of 960 gm instead of 1 kg. What is his gain percent?

He gives only 960 gm while charging for 1000 gm, so the error is 40 gm. Using the false-weight formula, $\text{Gain \%}=\frac{\text{Error}}{\text{True value}-\text{Error}}\times 100=\frac{40}{960}\times 100=4\frac{1}{6}\%$. Hence the gain is $4\frac{1}{6}\%$.