The standard deviation of $15$ terms is $6$ and each item is decreased by $1$. Then the standard deviation of new data is?

for two data sets,each of size 5, the variances are given to be 4 and 5 andthe correspondings means are given to be 2 and 4 respectively. the variances of the combined data set is

The mode of the following numbers is ; 15,14,19,20,14,15,16,14,15,18,14,19,15,17,15.