Use nPr when you are choosing r items from a total of n items, and the arrangement/order IS important.
Use nCr when you are choosing r items from a total of n items, and the arrangement/order IS NOT important.
Q: Bob has ten different books, but must choose only three to bring along on his vacation. In how many ways can Bob choose three books?
Explanation: This is a combination (not permutation) problem, because the order of the books in Bob's bag does not matter. Find the nCr function on your calculator and enter '10 nCr 3'. On the TI-84 Plus, nCr is in MATH, PRB, 3.
Or, do this by hand using the formula for nCr, at the left.