Sampling with replacement - standard deviation
Can anyone help me with this problem?
Suppose there are N balls in total. At each sampling, a ball is randomly taken out, marked with a mark, and put back to the samples. If a ball is taken out for a second time during a sampling, just put back without doing anything. Let X denote the number of balls being marked after n repeated samplings. The expectation of X can be calculated as: N*(1 - (1 - 1/N)^n) . But how to calculate the standard deviation of X?