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Combinations and Permutations Practice Questions and Video Explanations

As a continuation of my introduction to Combinations and Permutations, here are some practice problems to test the strategies I demonstrated.  Also, I am trying out a new format. Instead of giving you practice problems and then writing out a half-page description for each one, I am simply going to give you the questions. But don’t worry – I am not leaving you in the lurch. For explanations, I am solving each of the questions – using my special approach, of course – in the video below.

 

Questions:

1. A committee is composed of a president, a vice president, and a treasurer. If six people are trying out for the three positions, how many different committees result?

(A) 20      (B) 40      (C) 60     (D) 105      (E) 120

2. A committee of three is to be chosen from six. How many unique committees result?

(A) 20      (B) 40      (C) 60      (D) 105      (E) 120

3. A committee is composed of a president, a vice president, and a treasurer. If five people are running for president, six people are running for vice president, and three are running for treasurer, how many unique committees result?

(A) 15 (B) 45 (C) 75 (D) 90 (E) 120

 

4. A jousting tournament requires that a team consist of two knights and two squires. The Merry Band is forming a team from five knights and three squires. How many different lineups can The Merry Band field?

(A) 10             (B) 13             (C) 15             (D) 30             (E) 120

 

5. A septet, a group composed of seven players, is made up of four strings and three woodwind instruments. If seven students try out for strings and seven different students try out for woodwinds, how many unique septets can result?

(A) 35      (B) 70      (C) 210      (D) 420     (E) 1225

About the Author

Chris Lele has been helping students excel on the GRE, GMAT, and SAT for the last 10 years. He is the Lead Content Developer and Tutor for Magoosh. His favorite food is wasabi-flavored almonds. Follow him on Google+!

27 Responses to Combinations and Permutations Practice Questions and Video Explanations

  1. Alice November 3, 2013 at 7:46 pm #

    Wow, I have been trying to understand these types of problems for weeks via the “unmentionable other GRE study book” method and just spending 10 minutes watching you explain and it all makes perfect sense. Thank you so much!

  2. Arpit October 27, 2013 at 2:47 am #

    Very very helpful, now i know how to identify between permutation and a combination.

    • Chris Lele
      Chris Lele October 28, 2013 at 2:13 pm #

      Great! It’s a very tough concept to wrap one’s head around, so I’m happy you got it :).

  3. S July 20, 2013 at 2:03 am #

    Hi Chris,

    Could you please explain the difference between questions 1. and 2. Both the scenarios seem the same to me!

    Thanks very much.

    • Chris Lele
      Chris Lele July 22, 2013 at 2:09 pm #

      The difference between 1 and 2 is subtle. Imagine that the president, vice-president and treasurer will stand on podiums of different height, much the way the gold, silver, and bronze medalists do at the Olympics. Being chosen as three people from the six doesn’t tell you on which podium they are standing. So once we’ve chosen three people from the six, we have to come up with a way to figure out the number of different ways the three can stand on the podiums.

      The second example, by contrast, doesn’t focus on doing anything different with the members once they are in the group. In other words, there are no podiums or designations (president, vice-president, etc.). Once the three people are in the group, they are in the group. We don’t care where they stand, sit, etc.

      Hope that helps!

      • S July 22, 2013 at 9:05 pm #

        Excellent! The analogy really makes it clear. Thank you!

        • Chris Lele
          Chris Lele July 23, 2013 at 10:31 am #

          You are welcome!

  4. Lydia July 13, 2013 at 1:25 pm #

    Hi Chris, my maths sucks! I found your explanations really really helpful!

    • Chris Lele
      Chris Lele July 15, 2013 at 2:15 pm #

      Thanks, I’m happy they were able to help you get over the hump :). Keep it up!

  5. Ari July 2, 2013 at 6:47 am #

    So is it safe to assume that any problem that asks “how many unique combinations” will be a Combinations problem? If so, are there any other ‘key’ words that we can use to identify P vs C?

    Thanks this is all really helpful!

    • Chris Lele
      Chris Lele July 2, 2013 at 10:59 am #

      Well…I don’t think most combination problems would explicitly state “how many unique combinations”. Also, I can imagine a question with a lock and, say, 3 different slots with single digit numbers. How many unique combinations on the lock? This is not a combinations problem.

      Typically, when the question asks about group/teams/lineups, then it is a combination problem. As long, that is, as the question is not asking about unique positions within the group/team/lineup (e.g., 1st place, 2nd place, etc.; president, vice-president, etc.).

      Hope that helps!

  6. Shamila May 4, 2013 at 2:43 pm #

    Thanks Chris! your blogs are really very helpful when approaching perm/comb questions. I hated this topic as I had nvr really seen it before GRE prep, but now at least I can approach them w/ somewhat understanding! I used to look at bunch of formulas and get really confused. Thanks to your videos and blogs.

    • Chris Lele
      Chris Lele May 7, 2013 at 1:04 pm #

      Hi Shamila,

      Great! I’m happy I was able to demystify this pesky question type. Once you get used to the dash method you don’t even have to worry about the formidable formula :)!

  7. Sammy May 29, 2012 at 11:57 am #

    Before viewing your blog along with these videos, I was disconcerted and almost ready to give up when it came to trying to solve these combination-permutation problems; but now I can firmly say that I am confident when approaching these questions on the practice GRE tests. Thanks Chris!

    • Chris Lele
      Chris May 29, 2012 at 1:48 pm #

      Great! I always love to hear comments like these. Most books out there overly complicate combinations and permutations. So I’m happy I elucidated instead of obfuscated :).

  8. Jayashree May 15, 2012 at 2:24 am #

    awesome method ..just tempted to solve many problems on this topic

    • Chris Lele
      Chris May 15, 2012 at 11:55 am #

      Great! The method definitely makes thing go much faster!

  9. nora April 12, 2012 at 5:37 pm #

    Oh my God! lifesaver… advanced pre-calc. test tomorrow, and i was literally tearing up because I was so frustrated. I finally understood this, and used this as a test to see how well I knew it. Needless to say, the dash method is my backup to see if I did it right. Thanks! you’re great!

    • Chris Lele
      Chris April 13, 2012 at 6:04 pm #

      Nora,

      You are welcome :). Good luck on the test!

      • nora April 13, 2012 at 6:52 pm #

        thanks! I thought it was super hard but thats because of the geometric series stuff.. i ended up using your method for all of it..:)

        • Chris Lele
          Chris April 16, 2012 at 11:53 am #

          Great – I am happy to hear my method was helpful!

  10. Trishla December 19, 2011 at 7:07 pm #

    Thank you sooo much, it was very easy to comprehend! I’ve always dreaded these and you made them easier.

    • Chris Lele
      Chris December 20, 2011 at 2:08 pm #

      You’re welcome! I am always happy to hear that from students, because permutations/combinations doesn’t need to be intimidating.

      • akshara January 3, 2012 at 2:13 am #

        your videos are really helpful,thank u…….

        • Chris Lele
          Chris January 3, 2012 at 2:29 pm #

          Great! I am happy they were helpful!

  11. zafrin October 23, 2011 at 4:48 pm #

    very very helpful. thank u sooooooo much

    • Chris Lele
      Chris October 24, 2011 at 12:27 pm #

      Thanks Zafran!

      I know how confusing and frustrating this topic can be…I am happy the video made things easier. Indeed over the years I’ve turned many students into combinations/permutations fans. One student started doing them for fun once I showed him this method!


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