Permutations

Expert replies

crackgmat007: Legendary Member; Posts: 882; Joined: Fri Feb 20, 2009 2:57 pmFollowed by: 1 member; Thanked: 15 times; GMAT Score:690

by crackgmat007 » Wed May 06, 2009 9:39 am

There are 5 cars to be displayed in 5 parking spaces with all the cars facing the same direction.
Of the 5 cars, 3 are red, 1 is blue, and 1 is yellow. If the cars are identical except for color, how
many different display arrangements of the 5 cars are possible?
A. 20
B. 25
C. 40
D. 60
E. 125

Is C not the correct answer? I got 20 as the number of arrangements if the cars are displayed in one direction. But if we reverse the direction of all cars, the total arrangements will be 20*2=40. Did I assume beyond what is stated in the problem?
OA – A

Last edited by crackgmat007 on Sun May 10, 2009 9:31 pm, edited 1 time in total.

Like this post

Source: Beat The GMAT — Problem Solving | Wed May 06, 2009 9:39 am

DanaJ: Site Admin; Posts: 2567; Joined: Thu Jan 01, 2009 10:05 amFollowed by: 550 members; Thanked: 712 times; GMAT Score:770

by DanaJ » Wed May 06, 2009 10:15 am

You are right, 20 is the right answer. You are not supposed to consider the cars facing in different ways, since that's exactly why you have that statement in there:

all the cars facing the same direction

That basically tells you that they share the direction and that you shouldn't worry about it. Just "worry" about the colors!

Like this post

apple100: Senior | Next Rank: 100 Posts; Posts: 55; Joined: Wed Mar 18, 2009 9:56 am; Thanked: 2 times

by apple100 » Sun May 10, 2009 6:24 am

DanaJ wrote:You are right, 20 is the right answer. You are not supposed to consider the cars facing in different ways, since that's exactly why you have that statement in there:

all the cars facing the same direction

That basically tells you that they share the direction and that you shouldn't worry about it. Just "worry" about the colors!

How did you get 20?

Like this post

Svedankae: Master | Next Rank: 500 Posts; Posts: 101; Joined: Tue Apr 07, 2009 12:30 am; Thanked: 2 times; GMAT Score:750

by Svedankae » Sun May 10, 2009 12:45 pm

It's 5! / 3!*1!*1!

Like this post

apple100: Senior | Next Rank: 100 Posts; Posts: 55; Joined: Wed Mar 18, 2009 9:56 am; Thanked: 2 times

by apple100 » Sun May 10, 2009 2:37 pm

Svedankae wrote:It's 5! / 3!*1!*1!

what is the formula for these questions?

Like this post

crackgmat007: Legendary Member; Posts: 882; Joined: Fri Feb 20, 2009 2:57 pmFollowed by: 1 member; Thanked: 15 times; GMAT Score:690

by crackgmat007 » Sun May 10, 2009 9:35 pm

Factorial of total # of cars/Factorial of cars that are identical.

Since there are 5 cars, factorial will be 5!
3 cars are red in color, hence 3!

5!/3! gives us 20. Hope this helps

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review