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A level statistics: Combinations question- please help

Been stuck on this for over an hour now, don't know what I'm doing wrong. the answer is 2/35,can anyone explain why?

A maths class includes 7 girls and 8 boys, including Tom and Anna. The teacher must choose a team of 4 students to go to a maths competition. What is the probability that the team includes both Tom and Anna?
Original post by Jmisteradams
...


A viable method is to consider the number of permutations.

Good luck. :smile:
Original post by lazy_fish
A viable method is to consider the number of permutations.

Good luck. :smile:


I did 15C4, and found that the total number of possible selections is 1365, any idea what I need to do next? Thank you for your help.
Original post by Jmisteradams
I did 15C4, and found that the total number of possible selections is 1365, any idea what I need to do next? Thank you for your help.


If Tom and Anna are already selected then you just want two more from the remaining 13. So how many selections does that give you?
(edited 9 years ago)
Original post by brianeverit
If Tom and Anna are already selected then you just want two more from the remaining 13. So how many selections does that give you?


ohhhh, 13C2 = 78 = 78/1365 = 2/35!! I kept doing (7C1 X 8C1 X 13C2), didn't realise Tom and Anna was a given.

THANK YOU SO SO SO MUCH!! :biggrin::biggrin:
(edited 9 years ago)
Original post by Jmisteradams
ohhhh, 13C2 = 78 = 78/1365 = 2/35!! I kept doing (7C1 X 8C1 X 13C2), didn't realise Tom and Anna was a given.

THANK YOU SO SO SO MUCH!! :biggrin::biggrin:


You're welcome
What exam board is this?
Original post by Super199
What exam board is this?


OCR, module s1 :smile:

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