Question

How do you calculate combinations on the TI-84?

Answer 1

I use a `TI-83+` but the procedure will be the same.

The procedure is called `nCr` and it means:
From a group of `n` you select `r` while the order is not important.

Say you want to choose 3 students as class-representatives from a class of 25:

Recipe
Type in `25`
Use the `MATH`-button and move to the `PRB`-menu (far right)
Choose `nCr` and `enter`
Type in `3` and `enter`
The answer (`2300`) will be the number of combinations.

BTW:
If the order is important (1st=president, 2nd=secretary, 3d=member), then you use `nPr` (=number of permutations) from the same menu (answer should be `13800`)

The ratio between the two is the number of different orders that this selection can be in. (in this case `3! =6`).
In other words: `nPr= nCr *r!`
Where `n` number to choose from, and `r` number to be chosen.