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## A survey was conducted to find out how many people in...

tagged by: swerve

This topic has 1 expert reply and 2 member replies

### Top Member

swerve Master | Next Rank: 500 Posts
Joined
29 Oct 2017
Posted:
179 messages

#### A survey was conducted to find out how many people in...

Thu Dec 07, 2017 10:28 am
A survey was conducted to find out how many people in a housing colony of 144 residents could swim, dance a drive a car. It was found that the number of people who could not swim was 89, the number of people who could not dance was 100 and that the number of people who could not drive a car was 91. If the number of people who could do at least two of these things, was found to be 37 and the number of people who could do all these things was found to be 6, how many people could not do any of these things?

A) 17
B) 23
C) 29
D) 35
E) 50

The OA is D.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.

### Top Member

GMATWisdom Master | Next Rank: 500 Posts
Joined
29 Nov 2017
Posted:
100 messages
14
Sat Dec 09, 2017 11:46 am
swerve wrote:
A survey was conducted to find out how many people in a housing colony of 144 residents could swim, dance a drive a car. It was found that the number of people who could not swim was 89, the number of people who could not dance was 100 and that the number of people who could not drive a car was 91. If the number of people who could do at least two of these things, was found to be 37 and the number of people who could do all these things was found to be 6, how many people could not do any of these things?

A) 17
B) 23
C) 29
D) 35
E) 50

The OA is D.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.
Got a message to provide an alternative method to solve the problem.

So here it is, using set theory.

Let the set of people who can swim be A, the set of people who can dance be B and the set of people who can drive be C. The total number of people i.e. Universe set (U) in the figure below is 144.

ttps://postimg.org/image/e44tsjr2t/" target="_blank">

We have to find (AâˆªBâˆªC)^c where (AâˆªBâˆªC)^c is the complement of (AâˆªBâˆªC)
(AâˆªBâˆªC)^c = U- (AâˆªBâˆªC) = 144 - (AâˆªBâˆªC) ...................... equation 1.
We know that A^c = 89 or 144 - A = 89. i.e. A = 144 - 89 = 55. Similarly, B = 44 and C = 43 ...................equation 2.

In set theory, number of people in two or more sets is (A âˆ© B) + (B âˆ© C) + (C âˆ© A) -2 (A âˆ© B âˆ© C) and we know that this number is 37.

We also know that (A âˆ© B âˆ© C) = 6 ...................equation 3.

Hence (A âˆ© B) + (B âˆ© C) + (C âˆ© A) = 37 + 2 * 6 = 49...................equation 4.

Putting all the values from equation 2, 3 and 4 the above equation, (AâˆªBâˆªC) = 55 + 44+ 43 - 49 + 6 = 109 ...................equation 5.
Using equation 1 and 5
(AâˆªBâˆªC)^c = 144 - 109 = 35.

Hence D.

Thanked by: swerve

### Top Member

GMATWisdom Master | Next Rank: 500 Posts
Joined
29 Nov 2017
Posted:
100 messages
14
Fri Dec 08, 2017 9:34 am
swerve wrote:
A survey was conducted to find out how many people in a housing colony of 144 residents could swim, dance a drive a car. It was found that the number of people who could not swim was 89, the number of people who could not dance was 100 and that the number of people who could not drive a car was 91. If the number of people who could do at least two of these things, was found to be 37 and the number of people who could do all these things was found to be 6, how many people could not do any of these things?

A) 17
B) 23
C) 29
D) 35
E) 50

The OA is D.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.
let us take number of people who could only swim = a
...........................................................................................drive = b
...........................................................................................dance = c
...........................................................................................swim and drive = d
............................................................................................drive and dance= e
...........................................................................................dance and swim = f
..................................................................................not do any of the items= Z
number of people who know swiming+driving+dancing = 6
......................................................... at least two items = 37

therefore a+b+c+d+e+f+Z+6= 144

and d+e+f = 37 - 6 = 31

therefore a+b+c+ 31 +Z+6= 144

or a+b+c = 107- Z

those who could not swim = 89
therefore b+c+e+ Z = 89
Similaly a+c+f+ Z = 91
and a+b+d+ Z = 100
Adding we get 2(a+b+c) + (d+e+f) + 3Z =280
or 2(107-Z) + 31 + 3Z = 280

This gives Z = 280 - 214-31 = 35

Hence D is the correct answer

Thanked by: swerve

### GMAT/MBA Expert

Matt@VeritasPrep GMAT Instructor
Joined
12 Sep 2012
Posted:
2640 messages
Followed by:
113 members
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Target GMAT Score:
V51
GMAT Score:
780
Thu Dec 07, 2017 5:28 pm
When we've got three groups A, B, and C with some overlap, we can say that

Total = A + B + C + (people in no groups) - (people in exactly two groups) - 2*(people in exactly three groups)

We're told that the total = 144, A = (144 - 100), B = (144 - 89), and C = (144 - 91). (Remember that A, B, C are the people who CAN do each of the three activities, and Can = Total - Can't.)

We're also know that (people in exactly three groups) = (people in at least two groups) - (people in exactly two groups), so people in exactly three groups = 6 and people in exactly two groups = 31.

From there, our equation is

144 = (144 - 100) + (144 - 89) + (144 - 91) + (people in no groups) - 31 - 2*6

which, after some tedious arithmetic, gets us

35 = people in no groups

Thanked by: swerve
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