How can i use casio 991ms to solve all statistics questions

First off you want you calculator in SD (standard deviation) mode to do this hit mode twice and then 1.
next it is good practice to start off by clearing the memory to do this hit shift, CLR,1 and then = this will clear statistical memory
to enter values into your data set , you enter the value, then press the DT (M+) button. (note: pressing DT repeatedly will impute the same value multiple times)
once you have finished pluging in your numbers your calculator can give you statistical information about the data set.
hitting shift then 1(S-SUM) gives you three options: 1 the sum of the squares of each value 2 the sum of each value 3 the number of terms in the data set hitting shift then 2 (S-VAR) gives you three more options 1 x bar (the mean) 2 x sigma n (the population standard deviation) 3 x sigma n-1 (the sample standard deviation)
you can use these tools to solve alot of statistical problems i hope this helped
Source: http://support.casio.com/pdf/004/fx115MS_991MS_E.pdf page 28 of he pdf