If on an average 9 ships out of 10 arrive safely to ports, find the mean and S.D. of ships returning safely out of a total of 500 ships.
Let p be the probability of a ship returning safely to ports, so
Given, n = 500
Mean = np
= 
= 450
∴ Mean = 450
Standard deviation = 
= 6.71
So, mean = 450, standard deviation = 6.71
10