Posted by **ana** on Sunday, March 7, 2010 at 7:38pm.

At Maryland Middle School, there are 9 mathematics classes. If two mathematics classes are selected for a school competition, what is the number of possible outcomes?

- math -
**Damon**, Sunday, March 7, 2010 at 7:49pm
combinations of 2 out of 9

C(n,r) = n!/[(n-r)! (r!) ]

= 9! /[ 7! * 2! ]

= 9*8 *7! / [ 7! * 2]

=9*4

= 36

- math -
**Damon**, Sunday, March 7, 2010 at 7:53pm
Or use Pascal's triangle

row 9 is

1 9 36 84 126 126 84 36 9 1

so r = 0 --> 1

r = 1 = 9 of course

r = 2 = 36

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