# How do you calculate combinations on the TI-84?

Feb 9, 2015

I use a $T I - 83 +$ but the procedure will be the same.

The procedure is called $n C r$ 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 $M A T H$-button and move to the $P R B$-menu (far right)
Choose $n C r$ and $e n t e r$
Type in $3$ and $e n t e r$
The answer ($2300$) will be the number of combinations.

BTW:
If the order is important (1st=president, 2nd=secretary, 3d=member), then you use $n P r$ (=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.