BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

when to use permutation?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 43
Joined: Wed Jan 21, 2009 2:24 pm
Thanked: 1 times

when to use permutation?

by r_walid » Thu Jan 22, 2009 11:13 pm
for example, if there are 5 balls and we need to pull 3 out, assumin that each color is different but the order doesnt matter.what is the # of possibilities?
this is a typical combination right? 5C3.

but my question is this. why??? assume 3 empty spots, the first would have 5 possible, second is 4 and last 3. so combination is 5*4*3. this is diff. from above, can someone tell me why my method is wrong? why should i use combination formula and not my method?
Top
Source: — Problem Solving |
User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Fri Jan 23, 2009 1:08 am
Your method is an arrangement. Using that formula you get the answer if the order mattered, but you clearly stated that it doesn't. Let me just try and give you an example:
1. Say you have the balls marked a, b, c, d and e. Then since the order doesn't matter, you get that case a b c = case a c b and therefore this case is not counted twice, but just once.
2. If you use your way of looking at things, the two cases outlined before are considered different, therefore each will be counted once, giving you twice as many options as compared to the first case.
Top
Senior | Next Rank: 100 Posts
Posts: 43
Joined: Wed Jan 21, 2009 2:24 pm
Thanked: 1 times

by r_walid » Fri Jan 23, 2009 7:00 am
Thank Dana,
however, I knwo that permutation is the case when to use it when the order does matter.
so in this case, why isn't arrangement possible?
I mean, can you give me the same example with slight changes that would illustrate the following?
1-a case when arrangement is used
2- a case when permutation is used
3- a case when combination is used.

Thank you so much for your help
Top
User avatar
Site Admin
Posts: 2567
Joined: Thu Jan 01, 2009 10:05 am
Thanked: 712 times
Followed by:550 members
GMAT Score:770

by DanaJ » Fri Jan 23, 2009 10:45 am
Permutations are used when the number of balls doesn't matter... like for instance the number of cases for extracting whichever number of balls (1 to 5), but when the order does make a difference (meaning when a b c is not considered the same as b a c).
I'm sorry, I just cannot think of examples right now... I myself HATE permutations and probabilities... And I have not read an example of the sort in years. Maybe someone else can help you, or maybe the ball example is enough for you. I'm really sorry...
Top
Post new topic Post Reply
• Page 1 of 1