Math
posted by Rena on .
Suppose that the U.S. Senate consisted of 57 Republican and 43 Democratic Senators. How many different 12person committees could be formed with 8 Republican senators and 4 democratic senators?

C(57,8) x C(43,4) = ....

Is the answer 2508.52
and if so, how do I explane the whole solution. I am acting as a elementary teacher and have to discuss how to work each part out. 
the answer has to be a whole number, you can't have part of a committee, (although with most governments that is obviously possible)
C(n,r) or _{n}C_{r}
is defined as n!/(r!(nr)!)
and gives us the number of combinations of r things from n possibilites
e.g. given A, B, C, D, how many combination of 2 letters can we form
we could list them AB, AC, AD, BC, BD, and CD
notice this would be C(4,2) = 4!/(2!2!)
= 4x3x2x1/(2x1x2x1) = 24/4 = 6
a number like C(57,8) is quite large
around 1.6 billion
and to do it on your calculator by just using the definition, will overload your calculator.
Look for a key labeled something like
_{n}C_{r}
test it on C(4,2)
enter 4
press 2nd _{n}C_{r}
press 2
=
you should get 6
btw, the answer to your question is usually left in the notation I used, to express it in its full 15 digit number is rather futile.
Are you doing this in elementary school?? Wow
where?