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Easy probability question

There are 20 questions.

8 questions will appear in an exam.

I must choose 3 of those 8 questions to answer in the exam.

I have 2 weeks to prepare.




What is the minimum amount of questions I must revise in order to ensure I can answer 3 of these questions in the exam?
(edited 4 years ago)
Reply 1
Avatar for googie3
googie3
15
The minimum number of questions you must revise over the two weeks is 20 since any 8 of the 20 questions can appear and you can do any 3 of them.
(edited 4 years ago)
Reply 2
Avatar for Notnek
Notnek
21
Original post by Maths23xp
There are 20 questions.

8 questions will appear in an exam.

I must choose 3 of those 8 questions to answer in the exam.

I have 2 weeks to prepare.




What is the minimum amount of questions I must revise in order to ensure I can answer 3 of these questions in the exam?

I think you can work this out yourself. E.g. if you revise 10 of the questions then all 8 questions could be ones that you haven't revised. So that wouldn't guarantee it.

What about 11, 12, etc.?
Reply 3
Avatar for Maths23xp
Maths23xp
OP
7
Original post by googie3
The minimum number of questions you must revise over the two weeks is 20 since any 8 of the 20 questions can appear and you can do any 3 of them.


I am sure that is not right.

I think the answer is 15.

If I revise 15 questions - and 5 of the questions I HAVE NOT revised appear in the exam, that will leave 3 questions left which I will have revised (8 questions in total).
Reply 4
Avatar for Maths23xp
Maths23xp
OP
7
Original post by Notnek
I think you can work this out yourself. E.g. if you revise 10 of the questions then all 8 questions could be ones that you haven't revised. So that wouldn't guarantee it.

What about 11, 12, etc.?


I think the answer is 15.

If I revise 15 questions - and 5 of the questions I HAVE NOT revised appear in the exam, that will leave 3 questions left which I will have revised (8 questions in total). It is these 3 questions which I must answer.
Reply 5
Avatar for Notnek
Notnek
21
Original post by googie3
The minimum number of questions you must revise over the two weeks is 20 since any 8 of the 20 questions can appear and you can do any 3 of them.

That's not right. E.g. if they revise 19 questions then even if that 1 question that they didn't revise comes up in the 8, they only have to choose 3 so it would be guaranteed that they have revised enough questions.
Reply 6
Avatar for Notnek
Notnek
21
Original post by Maths23xp
I think the answer is 15.

If I revise 15 questions - and 5 of the questions I HAVE NOT revised appear in the exam, that will leave 3 questions left which I will have revised (8 questions in total). It is these 3 questions which I must answer.

Yes that's correct if you want to guarantee that you have revised enough questions. If you want to ensure with a high percentage (as opposed to 100%) then the question becomes a bit harder :smile:

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