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Combination/probability question (High School level)

Micky1964

New member
Sep 15, 2021
4
Hi all. I'm doing a counting/probability problem here.
I can see how the correct answer for part b is arrived at by counting the number of combinations for "2 girls" then counting the number of combinations for "3 girls" , and adding these results.
But I've attached my first way of approaching it, which is wrong. Wondering if anyone can tell me why my approach doesn't give the correct answer. I know it's probably something obvious that I'm missing.
Thanks
 

Attachments

romsek

Member
Mar 27, 2017
29
With your method groups with 3 girls get counted 3 times each rather than once each.
 

Micky1964

New member
Sep 15, 2021
4
With your method groups with 3 girls get counted 3 times each rather than once each.
Thank you for replying. I can see that makes sense now.

So the best approach with these types of problems is to treat each situation separately? ie. find how many ways you can have 2 girls and 1 boy, then find how many ways you can have 3 girls and 0 boys, then add the 2 figures?

Mick
 

romsek

Member
Mar 27, 2017
29
Thank you for replying. I can see that makes sense now.

So the best approach with these types of problems is to treat each situation separately? ie. find how many ways you can have 2 girls and 1 boy, then find how many ways you can have 3 girls and 0 boys, then add the 2 figures?

Mick
It's a bit difficult to cite any approach as "best" given the variety of problems that exist but generally separating the cases helps ensure you won't multiply count specific situations.
 

Country Boy

Well-known member
MHB Math Helper
Jan 30, 2018
881
I think it odd that choosing students "to represent the class in a debate" would be done at random!
 

topsquark

Well-known member
MHB Math Helper
Aug 30, 2012
1,296
I think it odd that choosing students "to represent the class in a debate" would be done at random!
You clearly never went to my High School.

-Dan
 

Country Boy

Well-known member
MHB Math Helper
Jan 30, 2018
881
Yeah, I dropped out in the fourth grade!