The most common JEE statistics problem: given mean and variance of n observations, one value was recorded incorrectly. Find the corrected statistics. Method: (1) From mean, find incorrect sum: sum = n * mean. (2) From variance, find incorrect sum of squares: sum($xi^2$) = n*(variance + $mean^2$). (3) Correct the sum: new sum = old sum - wrong + correct. (4) Correct sum of squares: new sum($xi^2$) = old sum($xi^2$) - $wrong^2$ + $correct^2$. (5) If the observation is removed (not corrected), use n-1 in the denominator. (6) Compute corrected mean and variance from new sums. This procedure works for any number of corrections — just adjust sums for each changed value. Always recover the raw sums first; never try to directly adjust the mean or variance.