Solution:
Provided: Two sets each of 20 observations, have the same standard derivation 5 . The first set has a mean 17 and the second a mean 
We now need to show that the standard deviation of the set obtained by combining the given two sets
As per the criteria given, for the first set
No. of observations, 
Standard deviation, 
And mean, 
For the second set, no. of observations, 
Standard deviation, 
And mean, 
It is known to us that the standard deviation for combined two series is
Substitute the corresponding values,
On simplifying we obtain
Take LCM and simplify,
Or, 
As a result, 
