Standard Deviation

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.

The average marks of 10 students in a class was 60 with a standard deviation 4 while the average marks of the other ten students was 40 with a standard deviation 6. If all the 20 students are taken together their standard deviation will be

The average marks of 10 students in a class was 60 with standard deviation 4. While the average marks of other 10 students was 40 with a standard deviation 6. If all the 20 students are taken together,their standard deviation will be