Random samples of size n were selected from binomial populat

Random samples of size n were selected from binomial populations with population parameters p given here. Find the mean and the standard deviation of the sampling distribution of the sample proportion p in each case. (a)    

n = 400, p = 0.7

Solution

The mean np=400*0.7=280

The standard deviation is:

(0.7*0.3)/400=0.022