Variance sigma^2 = sum(xi^2)/n - (x-bar)^2. This "sum of squares over n minus square of mean" form is far superior to the definitional formula sum(xi - x-bar)^2/n for computation. For frequency data: sigma^2 = sum(fi*xi^2)/N - (x-bar)^2. Remember: variance is always >= 0, equals 0 only when all observations are identical. Most JEE problems give sum(xi) and sum(xi^2) and expect you to use this formula directly.