Problems similar to the actual GMAT

[securitygeek]

A lot of people asked me what kind of probability problems I got on the actual exam. Since I signed a non disclosure agreement, I can't reveal the actual questions, but following are two permutation/combinations- probability problems similar to the ones I got on the real GMAT.

1. There is a set of characters, A B C D E F G H I J K. There are 4 character and 3 character codes to be made out of these characters. What is the ratio of the total number possibilities of the 4 character codes to the total possibilities of the 3 character codes?

2. There are 6 red balls and 3 blue balls in a bag. 2 red balls are taken out. What is the possibility that the next ball taken out will be a red ball? ( Yeah, they are that simple)

[prep2max]

1. 11c4*41 : 11c3*3!

2. 4/7

[haneen97]

Is it not the first one is a permutation? I think order matter since ABC is a different code that CBA.



I got 8:1

[securitygeek]

First one is a permutation problem. I thought it was a combination, but none of the answer choices matched my answer ( thank god). Then I did this the permutations way and my answer matched the answer choice

[rd_eastbay]

Thanks Security. Yes (1) is a permutation problem. The answer is 11P4 / 11P3 = 8.

[jatinet]

rd_eastbay, 11P4 I know how to solve combinations but not sure of permutations will appreciate if you could explain a bit about solving such permutaions

[rd_eastbay]

Sure. First the formula : nPk = n!/(n-k)!. I assume you already knew that but it bears repeating.

So 11P4 = 11!/7! = 11 x 10 x 9 x 8
And one has to use permutations instead of combinations because (as explained earlier), even if the same alphabets are picked (say A, B, C) they can be arranged in different ways. Ie whenever "order" matters, use permutations.

HTH.

Originally Posted by jatinet

rd_eastbay, 11P4 I know how to solve combinations but not sure of permutations will appreciate if you could explain a bit about solving such permutaions

[jatinet]

Thanks rd

[lego2401]

1- 11*10*9*8/11*10*9 = 8

2- 4/7

Yes, from my experience, this is pretty much the standard of prob/comb/perm Qs on the GMAT. No matter which range your in, it wont get more difficult than this. Don't work too much on these questions, you'll waste valuable time that can be used to enhance other skills for the GMAT. This is pretty much as hard as it gets.

[rd_eastbay]

Lego / Security,

Any examples of the algebra/number-thoery problems. While conceptually not hard, I find these real difficult to get right under time pressure. I am sure many other people feel the same way...

Originally Posted by lego2401

1- 11*10*9*8/11*10*9 = 8

2- 4/7

Yes, from my experience, this is pretty much the standard of prob/comb/perm Qs on the GMAT. No matter which range your in, it wont get more difficult than this. Don't work too much on these questions, you'll waste valuable time that can be used to enhance other skills for the GMAT. This is pretty much as hard as it gets.