Like permutations, combinations is a concept that unnecessarily frightens students: there are very few, if any combination questions, on a test that a student may have to take the SAT for an entire year to even see one of these question types (the SAT is administered eight times a year).
Why then even talk about combinations? Well, to make the announcement above (basically, don’t sweat it). But if you want to make sure you have all your math bases covered, then read on.
Rule #1 Combinations is about choosing
The ‘C’ in combinations equals ‘c’hoosing. A silly mnemonic perhaps, but one that will hopefully help you tell the difference between permutations and combinations.
Mark has a brown, white, blue, red, and black T-shirt. If he wants to pack two T-shirts for a weekend trip, then how many different T-shirts can he take with him?
Notice that, unlike a permutation question, Mark is not arranging his shirts in an order. He is choosing two to take with him. We do not care whether his red T-shirt is packed on top of his white T-shirt. This ordering (or arrangement) is key to a permutation problem but does not relate to a combination problem.
So now that we know we are dealing with a combination problem (after all this is a combination post), we need to use the following formula:
Notice how we can easily cancel out the 3 x 2 x 1 from both the numerator and the denominator. This leaves us with 5 x 4/2 = 10. Therefore, Mark can take a total of 10 shirts.
Don’t fret combinations. They are very likely to show up on the test, and when they do, they are no more difficult than the question above.
More from Magoosh
About Chris Lele
Chris Lele is the GRE and SAT Curriculum Manager (and vocabulary wizard) at Magoosh Online Test Prep. In his time at Magoosh, he has inspired countless students across the globe, turning what is otherwise a daunting experience into an opportunity for learning, growth, and fun. Some of his students have even gone on to get near perfect scores. Chris is also very popular on the internet. His GRE channel on YouTube has over 10 million views. You can read Chris's awesome blog posts on the Magoosh GRE blog and High School blog! You can follow him on Twitter and Facebook!
Leave a Reply
Magoosh blog comment policy: To create the best experience for our readers, we will approve and respond to comments that are relevant to the article, general enough to be helpful to other students, concise, and well-written! :) If your comment was not approved, it likely did not adhere to these guidelines. If you are a Premium Magoosh student and would like more personalized service, you can use the Help tab on the Magoosh dashboard. Thanks!