Suppose we want to choose 6 letters, without replacement, from 10 distinct letters.

If order matters (much like forming... a 4 digit number-maybe for a safe?), you will be computing "permutations". The formula for "r" items chosen from "n" items, where order matters is Npermutations = nPr = n!/ (n-r)!

Npermutations = 10P6 = 10!/(10-6)= 10*9*8*7*6*5 = 151200

When order doesn't matter (such as in dealing a poker hand, its the cards you end up with that matter not the order in which they are dealt into your hand) you will be computing combinations. The formula for the number of combinations of "r "items from "n" choices is Ncombinations = nCr = n!/((n-r)!r!)

Ncombinations = 10C6 = 10!/((10-6)!6!) = (10*9*8*7)/(4*3*2*1) = 210

Notice how there are far fewer combinations than permutations. This makes sense because if you think about how 6 colors {red, blue, green, white, black, yellow} can be arranged, there are 6!=720 ways to arrange these 6 colors if order matters. If order doesn't matter, this set of 6 colors counts only once.
Same thing with numbers :)

Hope this helped, brainliest is always appreciated! :)

Have a nice day CX
~Mitsuna