TITLE: How do I know when to use nCr button and when to use nPr button on my calculator in which situation?
QUESTION [11 upvotes]: 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?

REPLY [5 votes]: 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.