1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Unique combinations in a set.(+)

  1. Jan 21, 2014 #1
    Hey there!

    I will start of with saying I´m not very good at English when considering mathematical
    terms, neither an expert in Math.

    My question goes as this:

    I have a set of 1000 questions - which will be given in rounds with a set of 10.
    So every round, you get 10 questions out of the 1000.

    We want to change out 5 questions in every round.
    So we want to have not only to have the total number of combinations
    where 1 question is unique - but where a set of 5 is.

    Please, if the formulation is a bit misleading - i will try to clarify in a reply.
    As i said, this is even hard for me to question in the right way!

    - Thanks in advance
  2. jcsd
  3. Jan 21, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hey Solumsen! Welcome to PF! :smile:

    I don't understand …

    you want 10 questions, each of which has not been asked before …

    but that automatically means that any 5 of them will not have been asked before :confused:
  4. Jan 21, 2014 #3
    The number of unique sets of 5 would be 1000!/(5!*995!).
    The number of unique sets of 10 would be 1000!/(10!*990!).
    The number of unique ways of grouping all unique sets of 10 into two groups of 5 would be (10!/(2*5!*5!))(1000!/(10!*990!)).

    Since I can't quite figure out what the problem is, I don't know if any of this helps.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook