# Simple combination question .

1. May 20, 2012

### mtayab1994

1. The problem statement, all variables and given/known data

A class consists of 33 students 20 girls and 13 boys.

2. Relevant equations

We want to choose a team of 5 class representatives consisting of 3 girls and 2 boys.

What is the possibility of choosing the 5 people.

3. The attempt at a solution

Using the combination definition i got: nCr=nPr/r! I got total of 88920 combinations . Is that correct.

2. May 20, 2012

### tiny-tim

hi mtayab1994!
(that's 20C3*13C2)

yes

(is anything worrying you about that?)

3. May 20, 2012

### mtayab1994

Not really, I just haven't learned it yet in school that's why I'm just trying to make sure.

4. May 20, 2012

### Ray Vickson

What are the n and r are you using in this formula? Is n = 33? Is r = 5? If so, you are counting all the 5-person committees. Is that what the question asked?

RGV

5. May 20, 2012

### tiny-tim

Ray, see my post
… I did that to save you having to check!

6. May 20, 2012

### Ray Vickson

Actually, it is important for the OP to answer the questions and to think about why people ask them! You and I both know the answer; does the OP know it?

RGV

7. May 20, 2012

### tiny-tim

yes, because he got the right answer (which i was confirming) …

the other answer would have been 237336

8. May 20, 2012

### Ray Vickson

OK, but another point (that I hope the OP gets) is the merit of showing some intermediate steps. Some instructors/TAs require that, and perhaps some do not, but it is always a good idea. A common situation is when a student writes down the intermediate step correctly but then makes an arithmetic error (or copies the results incorrectly) and so gets an incorrect final numerical answer. If he/she just writes down the number, and it is wrong, the question gets 0, but if an intermediate step is written the question may (depending on policy, etc.) get part marks---the theory being that the student seems to know what he/she is doing but just made a simple arithmetic mistake. Of course, the student needs to fit the requirements to the syllabus, but being explicit and communicating effectively cannot do any harm.

Also: as to your point about the 'right answer': I have seen it happen a few times over the years that a student uses the wrong formula, makes an arithmetic error and for that reason gets the right answer!

RGV