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Blazer Sweater
American | x | d - y | a
European | c - x | y | b
c d

We have to find whether x/T < y/T. Essentially x < y

I have tried to build the matrix above . Not the prettiest though :)

Let T = a + b + c + d

Here we have

1. (a + c) / T = 5/6
Not sufficient as we need done know anything about b and d

2. (b + d) / T =3/ 5
Not sufficient as we need done know anything about a and c

Now, we can say that
a = x + d - y
b = y + c + x

Subtracting we get 2(y-x) = (b-a) + (d-c)

Subtracting 1 and 2 we get

(b+d)/T - (a+c)/T = 3/5 - 5/6

=> (b-a) + (d-c) = T(3/5 - 5/6)

Hence, we can safely say that (b-a) + (d-c) is negative. This implies 2(y-x) is negative
Hence x > y which suggest the answer to the DS question is a definite NO, and hence sufficient to answer with both the options.

Hence Option C is the answer.
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jcaguirre91
In a class, every student is either American or European. Every student is wearing a blazer or a sweater. If a student is selected randomly, is the probability of the student being a European wearing a sweater greater than that of his/her being an American wearing a blazer?

(1) The probability that a randomly selected student is either American or wearing a blazer is 5/6.

(2) The probability that a randomly selected student is either a European or wearing a sweater is 3/5.

You have to use a cheat code we have no time in solving 4 variable equation

First there are 4 possibilities either the student can be A or E and wearing B or S (symbols for the nation and dress)

We have to take aid of the fact 2 variable can be solved with 2 equation similarly since we are being askes about a particular sum we can integrate then into 1 variable

Therefore the equations can be only obtained through 2 possibilities that's combining 1 and 2 therefore IMO C
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In a class, every student is either American or European. Every student is wearing a blazer or a sweater. If a student is selected randomly, is the probability of the student being a European wearing a sweater greater than that of his/her being an American wearing a blazer?

You can make a 2 by 2 matrix or take variables.
There are four categories of people: \(T=E_b + E_s +A_b+ A_s\)
We are comparing the probability of two categories
Thus we are looking for: Whether \(\frac{E_s}{T}>\frac{A_b}{T}\)

(1) The probability that a randomly selected student is either American or wearing a blazer is 5/6.

\(\frac{E_b+A_s+A_b}{T}=\frac{5}{6}\)

\(1-\frac{E_b+A_s+A_b}{T}=1-\frac{5}{6}\)

\(\frac{T-(E_b+A_s+A_b)}{T}=\frac{6-5}{6}\)

\(\frac{E_s}{T}=\frac{1}{6}\)

Nothing about \(A_b\)

(2) The probability that a randomly selected student is either a European or wearing a sweater is 3/5.

\(\frac{E_b+E_s+A_s}{T}=\frac{3}{5}\)

\(1-\frac{E_b+E_s+A_s}{T}=1-\frac{3}{5}\)

\(\frac{T-(E_b+E_s+A_s)}{T}=\frac{2}{5}\)

\(\frac{A_b}{T}=\frac{2}{5}\)

Nothing about \(E_s\)



Combined
We can say \(\frac{2}{5}>\frac{1}{6}\)
Thus, \(P(A_b)>P(E_s)\)
Sufficient

C

Also, we can observe that the qs states a clear either/or condition. Hence these events are mutually exclusive. Hence the general formula of AuB= A+B-AnB will not be very useful, can rather be confusing. Since there is not AnB or so to say, any intersection for any case- American or European, Sweater or Blazer- American+Blazer/American+Sweater/European+Blazer/European+Sweater. No intersection at all.
In other words intersection of any 2 events is 0

Can be confusing if we simply try to apply the formula-
AuB= 1 (in this case) = A+B-AnB

The above will simplify as-
AuB= 1 (in this case) = A+B-0
Since AnB=0
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I think "and" condition is not very clearly mentioned in the question

Like 1/6 is the probability of being an European or warning a sweater & not "AND". Please explain how did we assume remaining to be European AND wearing sweater
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Why do we not use the P(AorB) dependent event formula? I mean the ans would be the same anway because it will be AB + AS + AB + BE -AB (A+B - both) - but just curious about why we did not use it! Bunuel
chetan2u
In a class, every student is either American or European. Every student is wearing a blazer or a sweater. If a student is selected randomly, is the probability of the student being a European wearing a sweater greater than that of his/her being an American wearing a blazer?

You can make a 2 by 2 matrix or take variables.
There are four categories of people: \(T=E_b + E_s +A_b+ A_s\)
We are comparing the probability of two categories
Thus we are looking for: Whether \(\frac{E_s}{T}>\frac{A_b}{T}\)

(1) The probability that a randomly selected student is either American or wearing a blazer is 5/6.

\(\frac{E_b+A_s+A_b}{T}=\frac{5}{6}\)

\(1-\frac{E_b+A_s+A_b}{T}=1-\frac{5}{6}\)

\(\frac{T-(E_b+A_s+A_b)}{T}=\frac{6-5}{6}\)

\(\frac{E_s}{T}=\frac{1}{6}\)

Nothing about \(A_b\)

(2) The probability that a randomly selected student is either a European or wearing a sweater is 3/5.

\(\frac{E_b+E_s+A_s}{T}=\frac{3}{5}\)

\(1-\frac{E_b+E_s+A_s}{T}=1-\frac{3}{5}\)

\(\frac{T-(E_b+E_s+A_s)}{T}=\frac{2}{5}\)

\(\frac{A_b}{T}=\frac{2}{5}\)

Nothing about \(E_s\)



Combined
We can say \(\frac{2}{5}>\frac{1}{6}\)
Thus, \(P(A_b)>P(E_s)\)
Sufficient

C
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Why do we not use the P(AorB) dependent event formula? I mean the ans would be the same anway because it will be AB + AS + AB + BE -AB (A+B - both) - but just curious about why we did not use it! Bunuel
chetan2u
In a class, every student is either American or European. Every student is wearing a blazer or a sweater. If a student is selected randomly, is the probability of the student being a European wearing a sweater greater than that of his/her being an American wearing a blazer?

You can make a 2 by 2 matrix or take variables.
There are four categories of people: \(T=E_b + E_s +A_b+ A_s\)
We are comparing the probability of two categories
Thus we are looking for: Whether \(\frac{E_s}{T}>\frac{A_b}{T}\)

(1) The probability that a randomly selected student is either American or wearing a blazer is 5/6.

\(\frac{E_b+A_s+A_b}{T}=\frac{5}{6}\)

\(1-\frac{E_b+A_s+A_b}{T}=1-\frac{5}{6}\)

\(\frac{T-(E_b+A_s+A_b)}{T}=\frac{6-5}{6}\)

\(\frac{E_s}{T}=\frac{1}{6}\)

Nothing about \(A_b\)

(2) The probability that a randomly selected student is either a European or wearing a sweater is 3/5.

\(\frac{E_b+E_s+A_s}{T}=\frac{3}{5}\)

\(1-\frac{E_b+E_s+A_s}{T}=1-\frac{3}{5}\)

\(\frac{T-(E_b+E_s+A_s)}{T}=\frac{2}{5}\)

\(\frac{A_b}{T}=\frac{2}{5}\)

Nothing about \(E_s\)



Combined
We can say \(\frac{2}{5}>\frac{1}{6}\)
Thus, \(P(A_b)>P(E_s)\)
Sufficient

C

The question directly gives you P(A or B), like "American or Blazer" = 5/6. It does not provide P(A), P(B), or P(A and B) separately - so how would you apply the formula, where exactly would you plug it in, and why would it help?
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Yes, but Bunuel, 1/6 is the probability of being European OR wearing a sweater & not "AND" - which is what the question asks - so how did we assume this?
Bunuel
ndwz
Why do we not use the P(AorB) dependent event formula? I mean the ans would be the same anway because it will be AB + AS + AB + BE -AB (A+B - both) - but just curious about why we did not use it! Bunuel
chetan2u
In a class, every student is either American or European. Every student is wearing a blazer or a sweater. If a student is selected randomly, is the probability of the student being a European wearing a sweater greater than that of his/her being an American wearing a blazer?

You can make a 2 by 2 matrix or take variables.
There are four categories of people: \(T=E_b + E_s +A_b+ A_s\)
We are comparing the probability of two categories
Thus we are looking for: Whether \(\frac{E_s}{T}>\frac{A_b}{T}\)

(1) The probability that a randomly selected student is either American or wearing a blazer is 5/6.

\(\frac{E_b+A_s+A_b}{T}=\frac{5}{6}\)

\(1-\frac{E_b+A_s+A_b}{T}=1-\frac{5}{6}\)

\(\frac{T-(E_b+A_s+A_b)}{T}=\frac{6-5}{6}\)

\(\frac{E_s}{T}=\frac{1}{6}\)

Nothing about \(A_b\)

(2) The probability that a randomly selected student is either a European or wearing a sweater is 3/5.

\(\frac{E_b+E_s+A_s}{T}=\frac{3}{5}\)

\(1-\frac{E_b+E_s+A_s}{T}=1-\frac{3}{5}\)

\(\frac{T-(E_b+E_s+A_s)}{T}=\frac{2}{5}\)

\(\frac{A_b}{T}=\frac{2}{5}\)

Nothing about \(E_s\)



Combined
We can say \(\frac{2}{5}>\frac{1}{6}\)
Thus, \(P(A_b)>P(E_s)\)
Sufficient

C

The question directly gives you P(A or B), like "American or Blazer" = 5/6. It does not provide P(A), P(B), or P(A and B) separately - so how would you apply the formula, where exactly would you plug it in, and why would it help?
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ndwz
Yes, but Bunuel, 1/6 is the probability of being European OR wearing a sweater & not "AND" - which is what the question asks - so how did we assume this?

P(American or Blazer) means the inclusive or, it includes all students who are American, wearing a blazer, or both. That’s why subtracting this from 1 gives the probability of the only group left out: European Sweater. So 1 - 5/6 = 1/6 is correctly used to find P(E_s), just as the question requires.
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