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Re: A survey was conducted to find out how many people in a housing colony [#permalink]
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Chaithraln2499 wrote:
Is this a formula to remember? Because I've never come across Venn formula when none of the events is included

Carcass wrote:
T=n(A)+n(B)+n(C)-n(Exactly two of the events)-2*n(All 3 Events)+n(None of the events)

T=144
n(A)=T-n(A')=144-89=55
n(B)=T-n(B')=144-100=44
n(C)=T-n(C')=144-91=53
n(Exactly two of the events)=n(At least 2 Events)-n(All 3 Events)=37-6=31
n(All 3 Events)=6

144=55+44+53-31-2*6+n(None of the events)
n(None of the events)=144-55-44-53+31+12=35

Ans: "D"


I guess above is a gmat question or similar.

Look at here for overlapping sets for the GRE. maybe above is a bit out of scope even though , in my view, would we worth to remember

https://gre.myprepclub.com/forum/gre-quant ... tml#p54136

regards
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Re: A survey was conducted to find out how many people in a housing colony [#permalink]
Carcass wrote:
T=n(A)+n(B)+n(C)-n(Exactly two of the events)-2*n(All 3 Events)+n(None of the events)

T=144
n(A)=T-n(A')=144-89=55
n(B)=T-n(B')=144-100=44
n(C)=T-n(C')=144-91=53
n(Exactly two of the events)=n(At least 2 Events)-n(All 3 Events)=37-6=31
n(All 3 Events)=6

144=55+44+53-31-2*6+n(None of the events)
n(None of the events)=144-55-44-53+31+12=35

Ans: "D"


Dear Carcass, but nowhere does it say that 37 people could do EXACTLY TWO of the things. It says "at least two", which means two or more. So I used the formula:

Total = A+B+C - (at least two) + all + neither and arrived at 23

Thank you.
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Re: A survey was conducted to find out how many people in a housing colony [#permalink]
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#not swim = 89 => #swim = 144-89 = 55
#not dance = 100 => #dance = 144 - 100 = 44
#not drive = 91 => #drive = 144 - 91 = 53

at least two = sum of two's + sum of three's = 37
sum of three's = 6
=> sum of two's = 31

total = S + D + Dr - (sum of two's ) - 2*(sum of three's) + none

144 = 55 + 44 + 53 -31 - 2*6 + none

=> none = 35

Answer is D.

Using the formula is a bad idea if you don't understand exactly when and how to use it. If you understand exactly when and how to use the formula, then you would find it too cumbersome to use it and will anyway prefer to reason out the answer!

In your formula, n(at least two of the events) is the sum of the intersection of the circles. This means each intersection includes the area where only two overlap and where all 3 overlap.
To check out the two formulas, check out this link: https://gmatclub.com/forum/a-school-has ... l#p1207266
After you check out the link, note that in your formula, n(at least two of the events) = (d + g) + (e + g) + (f + g)
whereas the 37 given to you in this question is (d + e + f + g)
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Re: A survey was conducted to find out how many people in a housing colony [#permalink]
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Carcass wrote:
After you check out the link, note that in your formula, n(at least two of the events) = (d + g) + (e + g) + (f + g)
whereas the 37 given to you in this question is (d + e + f + g)


That's where I had it all wrong! Now I'll try to figure out how to distinguish these two scenarios from the wording of the problem.

Thank you, dear Carcass, that was very helpful!
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