Operation Management - Standard Deviation

I am just confuse on how to compute the Standard Deviation for this problem. Plz help me.

I have to compute Standard Deviation for each line and that is where I dont know get it what to do.

So if some one can show me even how to do SD for "Activity" A then I can do rest.

Activity Immediate Predecessor---Optimistic Time--Most Likely Time--Pessimistic
A --------------------------------------10----------- 22---------------22