The mean life of a sample of 60 bulbs was 650 hours and the standard deviation was 8 hours. A second sample of 80 bulbs has a mean life of 660 hours and standard deviation 7 hours. Find the overall standard deviation.

Question

$\begin{array}{1 1}(A)\;8.94\\(B)\;5\\(C)\;7\\(D)\;15\end{array}$