-
williamthesituation
- Newbie | Next Rank: 10 Posts
- Posts: 8
- Joined: Mon Jan 25, 2010 4:12 pm
GMAT MATH MASTER's,
I am struggling to understand Combinatorics with Repetition. When there is a word like PIZZAZZ is is easier to understand the outcome is:
7!/4! = 7x6x5= 210
BUT, when you have a word problem I am having trouble deciding what to have in my denominator. One example from a book I have is:
If three of seven donkeys are selected for a trek up everest, how many different combinations of donkeys can be selected?
The answer is : 7!/(3!x4!) = 35
My question is why is the denominator 3!x4!, why not just 3! or 4!. This is the crux of my understand problem, I never know whether to include all the IN's as a factorial and the OUT's.
By the nature of these questions isn't one group always in and the rest out? So when do you decide to include either in the denominator? From review it seems sometimes you use both and sometimes you don't.
Somebody please help me see the light with this concept.
Thanks in advance.
(please do not spam this post to ring up your scorecard, unless you have the answer)
I am struggling to understand Combinatorics with Repetition. When there is a word like PIZZAZZ is is easier to understand the outcome is:
7!/4! = 7x6x5= 210
BUT, when you have a word problem I am having trouble deciding what to have in my denominator. One example from a book I have is:
If three of seven donkeys are selected for a trek up everest, how many different combinations of donkeys can be selected?
The answer is : 7!/(3!x4!) = 35
My question is why is the denominator 3!x4!, why not just 3! or 4!. This is the crux of my understand problem, I never know whether to include all the IN's as a factorial and the OUT's.
By the nature of these questions isn't one group always in and the rest out? So when do you decide to include either in the denominator? From review it seems sometimes you use both and sometimes you don't.
Somebody please help me see the light with this concept.
Thanks in advance.
(please do not spam this post to ring up your scorecard, unless you have the answer)
I know P&C and probability is confusing for many
