How do I know when to use nCr button and when to use nPr button on my calculator in which situation? How do I know when to use nCr button and when to use nPr button  on my calculator?
nCr= combinations I believe 
NPR= permutations
Is there a general rule I can use to figure out which one to use and when according to the given question?
 A: If you want to calculate ${}_nC_r=\frac {n!}{r!(n-r)!}$, use the nCr button.
If you want to calculate ${}_nP_r=\frac {n!}{(n-r)!}$, use the nPr button.
It is my strong belief that you should not be using either of these buttons until you have a clear understanding of why one of the above formulae is in fact something you want to calculate.
A: In brief, combinations should be used in situations where the order doesn't matter and permutations should be used where the order does. 
Say I am analysing a 49 ball lottery in which 6 balls are drawn. If I want to work out the odds of jackpot, then I need the number of different possible sets of 6 balls that can be drawn, which is 49C6. So the odds are 1/(49C6)
In contrast say I am a stage magician 'predicting' the lottery results ahead of selection, then I need to get the balls in the right order and the number of different ordered selections is 49P6, so the odds of getting this correct are 1/(49P6).
Incidentally this last number is about 0.00000001%, so if you do see a stage magician do this then you'd be justified in thinking there was something fishy about it.
A: In permutations, order counts.
So, if I wanted to know the number of ways to arrange a set of four unique books out of a total supply of 20 books, I'd use $20$ nPr $4$.  (Order counts in arranging books.)
In combinations, order doesn't count.
So, if I wanted to know the number of ways to make a team of $4$ people out of a total group of $20$ people, I'd use $20$ nCr $4$.  This is because order doesn't matter in choosing a team.
