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A certain business school has 500 students, and the law school at the : Multiple-choice Questions — Select One Answer Choice

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Re: A certain business school has 500 students, and the law school at the [#permalink]
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Ben and Lilian are siblings. Ben is studying Business and Lilian is studying Law.
- Possibility of selecting Ben = 1/500
- Possibility of selecting Lilian = 1/800

So the possibility of selecting this specific sibling pair is (1/500) * (1/800) = 1/400,000

Now imagine this concept with the total 30 sibling pairs.

(1/500) * (1/800) * 30 = 30/400,000 = 3/40,000
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Re: A certain business school has 500 students, and the law school at the [#permalink]
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1st - you need to pick on from any school, who has a sibling
P(l) = 15/500
P(b) = 15/800

2nd - you need to pick his pair sibling from the other school
P(b-l) = 1/800
P9l-b) = 1/500

3rd, you need to multiply resepectively ("AND" - for example - both P(l) and P(b-l) needed) and sum the result ("OR", one of the options needed)

1. 15/500*1/800 = 15/400,000
2. 15/800*1/500 = 15/400,000


1+2. = 15/400,000 +15/400,000 = 30/400,000 = 3/40,000
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Re: A certain business school has 500 students, and the law school at the [#permalink]
Cab you please explain the first part of this approach?
P(l) = 15/500
P(b) = 15/800

Why did you put 15 students in each school?
It talks about 30 pairs of siblings, so there are 60 students.
So each school has 30 students who have siblings in the other school, no?

Thank you for the help, I just confused by the 15 number.

Carcass wrote:
1st - you need to pick on from any school, who has a sibling
P(l) = 15/500
P(b) = 15/800

2nd - you need to pick his pair sibling from the other school
P(b-l) = 1/800
P9l-b) = 1/500

3rd, you need to multiply resepectively ("AND" - for example - both P(l) and P(b-l) needed) and sum the result ("OR", one of the options needed)

1. 15/500*1/800 = 15/400,000
2. 15/800*1/500 = 15/400,000


1+2. = 15/400,000 +15/400,000 = 30/400,000 = 3/40,000
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Re: A certain business school has 500 students, and the law school at the [#permalink]
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Expert Reply
Because they treat a couple as one

or you can think this way

probability of selecting sibling 30 students from b school 30/500
probability of selecting sibling 30 students from law school 30/800
probability that selected sibling students are members of the same sibling pair 1/30
(30/500)*(30/800)*(1/30) = 3/40000
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Re: A certain business school has 500 students, and the law school at the [#permalink]
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