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