It is currently 25 Jun 2017, 05:17

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Perm & Comb

 post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
Manager
Joined: 03 Sep 2005
Posts: 135
Perm & Comb [#permalink]

Show Tags

15 Sep 2005, 21:35
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Can anyone please provide clarification as to when we should use the permutation formula and when we should use the combination formula? I'm so confused and I only have 3 days until my test!

I was trying to solve this problem:

The telephone company wants to add an area code composed of 2 letters to every phone number. In order to do so, the company chose a special sign language containing 124 different signs. If the company used 122 of the signs fully and two remained unused, how many additional area codes can be created if the company uses all 124 signs?

I used the combination solution for this because we are trying to find the number of combos for 2 slots out of 122 and 124 different options. However, the solution just 124^2-122^2. I don't understand. Please help!
_________________

Life is not measured by the number of breaths you take, but by the number of moments that take your breath away.

Manager
Joined: 15 Jul 2005
Posts: 105

Show Tags

15 Sep 2005, 22:20
I think you may have over analyzed the problem!

It really asks how many more combinations are there if 124 characters are used, instead of 122. Since nothing is mentioned about repeating signs, we can assume repeating signs is allowed. 2 digits.

so for each digit 124 signs = 124*124
as opposed to 122 signs per digit = 122*122

which gives us 124^2-122^2
Manager
Joined: 15 Jul 2005
Posts: 105

Show Tags

15 Sep 2005, 22:27
Use permutation when order is of importance, and combination when it isn't. For example, when you count abc different from bca, permutation should be kept in mind. On the same note, if knowing abc has been selected is of importance, combination should be kept in mind.

Hope this helps!
Manager
Joined: 03 Sep 2005
Posts: 135

Show Tags

16 Sep 2005, 06:21
Overanalyzing is my middle name! Thanks for your input! If only I had your brain...

chets wrote:
Use permutation when order is of importance, and combination when it isn't. For example, when you count abc different from bca, permutation should be kept in mind. On the same note, if knowing abc has been selected is of importance, combination should be kept in mind.

Hope this helps!

_________________

Life is not measured by the number of breaths you take, but by the number of moments that take your breath away.

16 Sep 2005, 06:21
Display posts from previous: Sort by

Perm & Comb

 post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.