Hey there! Sign in to join this conversationNew here? Join for free

Combinations Help Watch

Announcements
    • Thread Starter
    Offline

    4
    ReputationRep:
    A box of 20 biscuits contains 4 different chocolate biscuits, 2 different oatmeal biscuits and 14 different ginger biscuits. 6 biscuits are selected from the box at random.
    (i) Find the number of different selections that include the 2 oatmeal biscuits.

    So I've tried using :
    Excluding oatmeal = 18C2, No restrictions = 20C6
    Selections = 20C6 - 18C2
    =20196

    But the correct answer is 18C4 = 3060

    I understand the answer's method, but could someone explain why my initial method is incorrect? Thanks
    Online

    8
    ReputationRep:
    (Original post by sourmango1)
    A box of 20 biscuits contains 4 different chocolate biscuits, 2 different oatmeal biscuits and 14 different ginger biscuits. 6 biscuits are selected from the box at random.
    (i) Find the number of different selections that include the 2 oatmeal biscuits.

    So I've tried using :
    Excluding oatmeal = 18C2, No restrictions = 20C6
    Selections = 20C6 - 18C2
    =20196

    But the correct answer is 18C4 = 3060

    I understand the answer's method, but could someone explain why my initial method is incorrect? Thanks
    For your approach to work, you would have to calculate it as unrestricted combinations - combinations with neither of the oatmeals - combinations with exactly one of the oatmeals. This would leave you with the combinations with two oatmeals as required. You have omitted the third element.

    (Also I'm guessing you mean 18C6 where you've written 18C2).
    • Thread Starter
    Offline

    4
    ReputationRep:
    (Original post by old_engineer)
    For your approach to work, you would have to calculate it as unrestricted combinations - combinations with neither of the oatmeals - combinations with exactly one of the oatmeals. This would leave you with the combinations with two oatmeals as required. You have omitted the third element.

    (Also I'm guessing you mean 18C6 where you've written 18C2).
    Ah yes I meant 18C6. Thank you I understand now.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Will you be richer or poorer than your parents?
    Useful resources

    Make your revision easier

    Maths

    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here

    Equations

    How to use LaTex

    Writing equations the easy way

    Student revising

    Study habits of A* students

    Top tips from students who have already aced their exams

    Study Planner

    Create your own Study Planner

    Never miss a deadline again

    Polling station sign

    Thinking about a maths degree?

    Chat with other maths applicants

    Can you help? Study help unanswered threads

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.