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Of the 45 students in a certain class, 30 joined math club,

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VP
Joined: 10 Jun 2007
Posts: 1438
Of the 45 students in a certain class, 30 joined math club, [#permalink]

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26 Sep 2007, 19:49
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Of the 45 students in a certain class, 30 joined math club, 25 joined physics club, and 20 joined biology club. The students who joined physics club also joined the math club. If 12 students joined both math and biology club, and 8 students joined all of the three clubs, how many students joined none of the three clubs?
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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26 Sep 2007, 20:06
1
KUDOS
ans = 7

my working is in the attachement.
Attachments

venn.jpg [ 14.76 KiB | Viewed 1958 times ]

Director
Joined: 11 Jun 2007
Posts: 915

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26 Sep 2007, 20:10
bkk145 wrote:
Of the 45 students in a certain class, 30 joined math club, 25 joined physics club, and 20 joined biology club. The students who joined physics club also joined the math club. If 12 students joined both math and biology club, and 8 students joined all of the three clubs, how many students joined none of the three clubs?

I am getting 7 for the answer using the Venn diagram

silly mistake.... grrrrrrrr
Intern
Joined: 15 Mar 2007
Posts: 45

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26 Sep 2007, 20:22
bkk145 wrote:
Of the 45 students in a certain class, 30 joined math club, 25 joined physics club, and 20 joined biology club. The students who joined physics club also joined the math club. If 12 students joined both math and biology club, and 8 students joined all of the three clubs, how many students joined none of the three clubs?

It's 7.

a. Math + Physics : 25
b. Math + Physics + Biology = 8
From a & b ==>
Only Physics = 0 & Only Physics + Math = 17 --> I

a. Math + Biology = 12
b. Math + Physics + Biology = 8 --> II

From a & b ==>
Only Math + Biology = 4 --> III

From I II & III : Only Math = 30 - ( 17 + 8 + 4 ) = 1

a. Physics + Biology : 0 ;
b. Math + Physics + Biology = 8 ;
c. Math + Biology = 4

a, b , c together ==> Only Biology = 20 - ( 12 + 8 ) = 8

Total : 45
No Clubs = k

k = 45 - ( Only Math + Only Physics + Only biology + ( Only Math & Physics) + ( Only Math & Biology ) + ( Only Physics & Biology) + ( All three)

==> k = 45 - ( 1 + 0 + 8 + 17 + 4 + 0 + 8 ) = 45 - 38 = 7
Intern
Joined: 15 Mar 2007
Posts: 45

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26 Sep 2007, 20:23
oops

Only biology is : 20 - ( 8 + 4 ) = 8
VP
Joined: 10 Jun 2007
Posts: 1438

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27 Sep 2007, 08:15

Total = 45

Math = 30
Physics = 25
Biology = 20

P&M = 25
M&B=12
P&B=8
P&B&M =8

30+25+20 - 25- 12 - 8 + 8 + None= 45
None = 7

However, I notice that you don't actually need to consider Physics students since all physics students are in Math. I wish I can draw, but basically, the physics circle is in math circle. Therefore, if you use just math and biology and m&b, then you should get the answer...
30+20 - 12 + None = 45
None = 7
Director
Joined: 03 May 2007
Posts: 872
Schools: University of Chicago, Wharton School

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27 Sep 2007, 12:07
ywilfred wrote:
ans = 7

my working is in the attachement.

nice work. btw, what you use to draw these pictures?
SVP
Joined: 28 Dec 2005
Posts: 1559

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27 Sep 2007, 19:15
bkk145 wrote:

Total = 45

Math = 30
Physics = 25
Biology = 20

P&M = 25
M&B=12
P&B=8
P&B&M =8

30+25+20 - 25- 12 - 8 + 8 + None= 45
None = 7

However, I notice that you don't actually need to consider Physics students since all physics students are in Math. I wish I can draw, but basically, the physics circle is in math circle. Therefore, if you use just math and biology and m&b, then you should get the answer...
30+20 - 12 + None = 45
None = 7

how do you know that p&b=8 ?
VP
Joined: 10 Jun 2007
Posts: 1438

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27 Sep 2007, 19:34
pmenon wrote:
bkk145 wrote:

Total = 45

Math = 30
Physics = 25
Biology = 20

P&M = 25
M&B=12
P&B=8
P&B&M =8

30+25+20 - 25- 12 - 8 + 8 + None= 45
None = 7

However, I notice that you don't actually need to consider Physics students since all physics students are in Math. I wish I can draw, but basically, the physics circle is in math circle. Therefore, if you use just math and biology and m&b, then you should get the answer...
30+20 - 12 + None = 45
None = 7

how do you know that p&b=8 ?

Since all P is in M, and you know that P&M&B = 8
This means P&M&B = P&B = 8
Director
Joined: 08 Jun 2007
Posts: 573

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27 Sep 2007, 21:16
ywilfred wrote:
ans = 7

my working is in the attachement.

As usual , gr8! .
And yes I did the same way and got 7 .
VP
Joined: 10 Jun 2007
Posts: 1438

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27 Sep 2007, 22:07
Just want to share strategy about cracking Venn diagram problem.

Basically, I don't know any Venn formula. What I do is I keep track of how many items has been counted. The total always consist of counting everything ONCE!

For example, if you are give two circle Venn diagram, you have total of A and B and the overlap (A&B). You know that if you add A and B, the overlap has been count twice, therefore, for formula will be:
Total = A+B - A&B

If you are given three circle Venn diagram of A, B, C, it is a bit more tricky, but same trick.
If you are given A&B, B&C, C&A, you know that A,B,C already count A&B&C three times. A&B count A&B&C one time. B&C count A&B&C one time. C&A count A&B&C one time. Therefore, to make everything count ONCE, you can come up with the following:

A+B+C - A&B - B&C - C&A + A&B&C = Total
A+B+C => count A&B&C three times
A&B => count A&B&C one time
B&C => count A&B&C one time
C&A => count A&B&C one time
From this, you know that the "counts" are all three and they cancel out and therefore, you must ADD A&B&C at the end to make everything count ONCE!

Now, some complicated problems mention of "exactly two". This means that you are given A&B, but it doesn't count A&B&C. Using the same trick, if you are given exactly A&B without A&B&C, B&C with out A&B&C, C&A without A&B&C, your formula becomes:

A+B+C - A&B - B&C - C&A - 2*A&B&C = Total

Why? Because A, B, C count A&B&C three times. You know that A&B, B&C, C&A don't count A&B&C at all. Therefore, to make everything count ONCE, you need to subtract 2*A&B&C to the equation.

If you know how to do the "count", you don't even need to remember any formula. I find a lot of problems that the formula will just make it confusing, though mostly tough problem. Using this trick, you won't make a mistake in Venn diagram type problem.

Using None is a cake walk at this point.

Cheers.
CEO
Joined: 29 Mar 2007
Posts: 2560

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27 Sep 2007, 23:06
Ugghhh Venn's one of biggest weaknesses.

How do u know B+P=0???
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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28 Sep 2007, 00:31
Fistail wrote:
ywilfred wrote:
ans = 7

my working is in the attachement.

nice work. btw, what you use to draw these pictures?

MS Powerpoint! Then cut and paste it as a picture =)
28 Sep 2007, 00:31
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