A combination is an arrangement of objects in which order does not matter.
There are 3 books on a shelf, a Math (M) book, a Physics (P) book, and a French (F) book. Suppose, you want to take any 2 books out.
The possibilities are:
Remove the repetitions PM, FM, FP as they are the same as MP, MF, and PF.
Now, we are left with only 3 possibilities and they tell us the combinations of books that you can probably take out.
Combination of r objects out of n objects is nCr = n!/[(n – r)! r!].
We can use this formula. Here, n! means 1x2x3x…xn.