Hi confused13,

For permutation and combination questions, you have some options when it comes to HOW you answer the question. Depending on the complexity of the situation and the "size" of the answer choices, you might need to do a lot of math OR you might be able to just "brute force" the situation.

For example, if you have 5 different markers and you want to figure out how many different combinations of 2 there are, how would you go about solve (order does not matter)? You could use the combination formula OR you could just write down all of the combinations. 8-year olds can solve this problem with crayons by just drawing it out. If you call the markers A, B, C, D and E, then you can name all the options relatively quickly.

In contrast, if it's 10 markers and you want to figure out how many different combinations of 4 there are, then you'd probably want to use the Combination Formula:

N!/[K!(N-K)!]

To that end, knowing the Combination Formula is a MUST.

Personally, I've never used the "formal" Permutation Formula to solve a GMAT question, but it doesn't bother me if someone else wants to. GMAT questions usually can be solved in a couple of different ways, so how you choose to do so is up to you.

GMAT assassins aren't born, they're made,
Rich

As Rich points out, knowing how to calculate combinations is a MUST.
One option is to use the combination formula: N!/[K!(N-K)!]
However, most combinations on the GMAT can be calculated quickly in our heads.
If anyone is interested, we have a free video that explains how to do that: https://www.gmatprepnow.com/module/gmat-counting?id=789

Cheers,
Brent

I'd also like to point out that most counting questions on the GMAT can be solved by applying the Fundamental Counting Principle (FCP). The FCP is easy to apply, and typically does not require any formulas. If a formula is required, it is almost always the combinations formula.

Here's an article I wrote on this topic: https://www.beatthegmat.com/mba/2013/07/ ... ons-part-i

For more information about the FCP, watch our free video: https://www.gmatprepnow.com/module/gmat-counting?id=775

Then you can try solving the following questions:

EASY
- https://www.beatthegmat.com/what-should- ... 67256.html
- https://www.beatthegmat.com/counting-pro ... 44302.html
- https://www.beatthegmat.com/picking-a-5- ... 73110.html
- https://www.beatthegmat.com/permutation- ... 57412.html
- https://www.beatthegmat.com/simple-one-t270061.html
- https://www.beatthegmat.com/mouse-pellets-t274303.html


MEDIUM
- https://www.beatthegmat.com/combinatoric ... 73194.html
- https://www.beatthegmat.com/arabian-hors ... 50703.html
- https://www.beatthegmat.com/sub-sets-pro ... 73337.html
- https://www.beatthegmat.com/combinatoric ... 73180.html
- https://www.beatthegmat.com/digits-numbers-t270127.html
- https://www.beatthegmat.com/doubt-on-sep ... 71047.html
- https://www.beatthegmat.com/combinatoric ... 67079.html


DIFFICULT
- https://www.beatthegmat.com/wonderful-p- ... 71001.html
- https://www.beatthegmat.com/ps-counting-t273659.html
- https://www.beatthegmat.com/permutation- ... 73915.html
- https://www.beatthegmat.com/please-solve ... 71499.html
- https://www.beatthegmat.com/no-two-ladie ... 75661.html
- https://www.beatthegmat.com/laniera-s-co ... 15764.html

Cheers,
Brent