If sum of squares of the values =3390, N=30 and standard deviation=7, find out the mean

Find the mean , variance and standard deviation :

The mean of four observations is 3. If the sum of the squares of these observations is 48 then their standard deviation is

A simple of 25 variates has mean 40 and standard deviation 5 and a second sample of 35 variates has mean 45 and the standard deviation 2. Find the mean and standard deviation of the two samples of variates , taken together.

Find the standard deviation by using 20 as assumed mean.

A sample of 35 observations has mean 80 men and standard deviation 4. A second sample of 65 observations has mean 70 and standard deviation =3. Find the combined mean and standard deviation.