Probability question?

Question

In probabily ,sometimes set and combination(C) is used to solve problem.In which time it is used and why?

Answer 1

well in my skool we take it as {} and {example:there is 5 white marble and 5 black marbles in a bag...u pick 2 randomly without replacing them in the bag.....wat is the probability that u get 2 white marbles==>this is a combination 2 the way it is solved is multiplying the the 2 combined probabilities
answer:pr:(white AND white)=(5/10)*(4/9)

now the set is this way....wat is the probability that u get 2 marbles of different colors in the first 2 picks===>set rule

answer:there is 2 combinations in which u can get 2 different colors there are {white then black} OR {black then white} u find the 2 combinations then ADD them
pr(WandB OR BandW)={(5/10)*(5/9)}+{(5/10)*(5/9)}

Answer 2

A combination is a set of objects in which order is not important :
C (n,m) = (n) (n-1) .... (n-m+1)/m!

The C is for combination
Otherwise when the order is important, the used letter is A