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# S1 probability question Watch

1. In a large college, 3/5 of the students are male, 3/10 of the students are left handed and1/5 of the male students are left handed.
A student is chosen at random.

(a) Given that the student is left handed, find the probability that the student is male.

I thought you'd have to divide 1/5 by 3/10 but according to the mark scheme, you don't. The mark scheme says to divide ( 3/5 x 1/5 ) by 3/10.

I don't understand this because if 1/5 represents male AND left handed, then why do you have to multiply 3/5 by 1/5 ?
2. 3/5 are male so 1/5 of the 3/5 are male, left handed. Which is 3/5 x 1/5. it is given that they are left handed which means 3/25 out of 3/10 which is 3/25 divded by 3/10
The 1/5 is just the proportion of the male students that are left handed. you can think of it as 20% of the male students are left handed which isnt 20% of the whole population which is what u did.
3. (Original post by fpmaniac)
3/5 are male so 1/5 of the 3/5 are male, left handed. Which is 3/5 x 1/5. it is given that they are left handed which means 3/25 out of 3/10 which is 3/25 divded by 3/10
The 1/5 is just the proportion of the male students that are left handed. you can think of it as 20% of the male students are left handed which isnt 20% of the whole population which is what u did.
Ooooh okay thanks

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