The mean and standard deviation of some data for the time taken to complete a test are calculated with the following results:Number of observations =$25$,mean =$18.2$ seconds, standard deviation $=3.25$ seconds. Further another set of $15$ observations $x_1,x_2,......x_5$ also in seconds is now available and we have $\sum\limits_{i=1} ^{15}x_i =279$ and $\sum \limits_{i=1} ^{15} x_i^2 =5524$Calculate the standard deviation based on all $40$ observations

Question

$\begin{array}{1 1}(A)\;1.99\\(B)\;3.873\\(C)\;8\\(D)\;1.25\end{array}$