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Each of 90 students participated at least 1 of the track try [#permalink]

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19 Feb 2013, 10:39

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Each of 90 students participated at least 1 of the track tryouts: High jump, long jump, 100 meter dash. If 20 students participated in high jump tryout, 40 students participated in the long jump tryout and 60 students participated in the 100 meter dash tryout, and if 5 students participated in all 3 tryouts, how many students participated in only two of these tryouts?

So the formula that we can work from is: Group A + Group B + Group C + Neither - Doubles - 2(Triples) = Total

So 20 + 40 + 60 + 0 - Doubles - 2(5) = 90

110-Doubles = 90

Doubles = 20

The above formula is critical. Although it is simple, it is important to really understand it. Once you do, groups will be much easier.

Happy Studies,

HG. _________________

"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land

We are asked how much are in only two tryouts that means what is (H and L only) + (L and 100m only) + (H and 100m only) Total = H + L + 100m - both + All + None 90 = 20 + 40 + 60 - both + 5 + 0 90 = 125 - both ----------> both = 35

35 = (H and L) + (L and 100m) + (H and 100m)

H and L only = H and L - all ------> H and L - 5 L and 100m only = L and 100m - all ------> L and 100m - 5 H and 100m only = H and 100m - all ------> H and 100m - 5

so (H and L only) + (L and 100m only) + (H and 100m only) = (H and L) + (L and 100m) + (H and 100m) - 15 ---------> 35 - 15 = 20.

Each of 90 students participated at least 1 of the track tryouts: High jump, long jump, 100 meter dash. If 20 students participated in high jump tryout, 40 students participated in the long jump tryout and 60 students participated in the 100 meter dash tryout, and if 5 students participated in all 3 tryouts, how many students participated in only two of these tryouts?

(A) 25 (B) 20 (C) 15 (D) 10 (E) 5

A venn diagram can show you exactly what you are supposed to do.

Attachment:

Ques4.jpg [ 18.31 KiB | Viewed 6316 times ]

The high jump circle has 20 students. It includes the pink region (only High jump), d, e and 5 (students who tried all 3) The long jump circle has 40 students. It includes the yellow region (only long jump), d, f and 5 (students who tried all 3) The 100 meter dash circle has 60 students. It includes the green region (only 100 meter), e, f and 5 (students who tried all 3)

So when you add 20 + 40 + 60, you get 120 which is 30 more than 90. Why? Because in 20+40+60, you have counted, d, e and f twice and 5 thrice. You need to subtract two times 5 and d+e+f once to get 90. 120 - 2*5 - (d+e+f) = 90 (d+e+f) = 20

This is how you get the formula: Group A + Group B + Group C + Neither - Doubles - 2(Triples) = Total _________________

Re: Each of 90 students participated at least 1 of the track try [#permalink]

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25 Feb 2013, 04:08

VeritasPrepKarishma wrote:

mun23 wrote:

Each of 90 students participated at least 1 of the track tryouts: High jump, long jump, 100 meter dash. If 20 students participated in high jump tryout, 40 students participated in the long jump tryout and 60 students participated in the 100 meter dash tryout, and if 5 students participated in all 3 tryouts, how many students participated in only two of these tryouts?

(A) 25 (B) 20 (C) 15 (D) 10 (E) 5

A venn diagram can show you exactly what you are supposed to do.

Attachment:

Ques4.jpg

The high jump circle has 20 students. It includes the pink region (only High jump), d, e and 5 (students who tried all 3) The long jump circle has 40 students. It includes the yellow region (only long jump), d, f and 5 (students who tried all 3) The 100 meter dash circle has 60 students. It includes the green region (only 100 meter), e, f and 5 (students who tried all 3)

So when you add 20 + 40 + 60, you get 120 which is 30 more than 90. Why? Because in 20+40+60, you have counted, d, e and f twice and 5 thrice. You need to subtract two times 5 and d+e+f once to get 90. 120 - 2*5 - (d+e+f) = 90 (d+e+f) = 20

This is how you get the formula: Group A + Group B + Group C + Neither - Doubles - 2(Triples) = Total

I get your method here..however can you please explain the problem mentioned below as well...

The 38 movies in the video store fall into the following three categories: 10 action, 20 drama, and 18 comedy. However, some movies are classified under more than one category: 5 are both action and drama, 3 are both action and comedy, and 4 are both drama and comedy. How many action-drama-comedies are there?

In this question, we are looking for those movies that are in all 3 categories.

The answer I am getting is -1 which is wrong

38=48-12(i got this 12 by adding 5+3+4)-2x therefore, 2=-2x

I get your method here..however can you please explain the problem mentioned below as well...

The 38 movies in the video store fall into the following three categories: 10 action, 20 drama, and 18 comedy. However, some movies are classified under more than one category: 5 are both action and drama, 3 are both action and comedy, and 4 are both drama and comedy. How many action-drama-comedies are there?

In this question, we are looking for those movies that are in all 3 categories.

The answer I am getting is -1 which is wrong

38=48-12(i got this 12 by adding 5+3+4)-2x therefore, 2=-2x

can you please help me fix this?

I am glad you asked this question. This is the reason I tell people to not use formulas in sets questions and to rely on their understanding of venn diagrams. The formula discussed above is used in case you know d, e and f i.e. no of people participating in two and only two events. In this question, the given numbers 5, 3 and 4 are not d, e, and f. They are the number of movies which fall into two groups as well as three groups. 5 movies which are both action and drama include those movies which are comedy as well i.e. which fall in all three categories.

In this case, the formula we use is P(A u B u C) = P(A) + P(B) + P(C) – P(A n B) – P(A n C) – P(B n C) + P(A n B n C) 38 = 48 - 12 + x x = 2

Think about this formula. P(A) includes P(A n B) which includes P(A n B n C). To avoid double counting in case of P(A), P(B) and P(C), you subtract P(A n B), P(A n C) and P(B n C) once but now you have subtracted P(A n B n C) thrice which means you haven't accounted for it at all. Hence, you add it back. _________________

I'm not a big fan of Venn diagrams and have found that for groups, these formulas, once you really practice them, are big time savers. In this case, the formula would be:

Singles + Doubles + Triples = Total

24 + 12 + Triples = 38

Triples = 2

This is just a derivative of the other formula. You have to play around with this to get the hang of it. OG 13 PS #178 is a good one to practice with.

Let me know if you have any questions. Happy studies.

HG. _________________

"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land

Re: Each of 90 students participated at least 1 of the track try [#permalink]

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25 Feb 2013, 09:23

I get Karishma's method .. Thanks

@ HerrGrau- how n here do you get your singles from tried the OG prob too thank u ..and do we have your debrief on the site as well ..your verbal is outstanding ..need to know how for that as well ..

You can find the singles by subtracting all of the overlaps from the fake total (I call it the fake total because it is the total that counts all of the movies that are in different categories). In this case we are told that there are 12 overlaps. You have to multiply that by 2 because each overlap is in two groups (for instance, there are 5 doubles in action and in drama so those 5 doubles take up 10 slots). So the singles are 48-24 = 24. There is no doubt that the "formula" approach to groups takes practice but it does work very well once you are comfortable with the formulas and how the different groups interact. Maybe a combined approach with Venn Diagrams would work for you.

Let me know if you need more advice on this.

HG.

PS: I am planning on writing a debrief but haven't yet found the time:( _________________

"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land

Re: Each of 90 students participated at least 1 of the track try [#permalink]

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30 Jul 2014, 17:10

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: Each of 90 students participated at least 1 of the track try [#permalink]

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31 Jul 2014, 03:01

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mun23 wrote:

Each of 90 students participated at least 1 of the track tryouts: High jump, long jump, 100 meter dash. If 20 students participated in high jump tryout, 40 students participated in the long jump tryout and 60 students participated in the 100 meter dash tryout, and if 5 students participated in all 3 tryouts, how many students participated in only two of these tryouts?

(A) 25 (B) 20 (C) 15 (D) 10 (E) 5

Refer diagram below

We require to find value of shaded region = a + b + c

Re: Each of 90 students participated at least 1 of the track try [#permalink]

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31 Jul 2014, 03:14

I get your method here..however can you please explain the problem mentioned below as well...

The 38 movies in the video store fall into the following three categories: 10 action, 20 drama, and 18 comedy. However, some movies are classified under more than one category: 5 are both action and drama, 3 are both action and comedy, and 4 are both drama and comedy. How many action-drama-comedies are there?

In this question, we are looking for those movies that are in all 3 categories.

The answer I am getting is -1 which is wrong

38=48-12(i got this 12 by adding 5+3+4)-2x therefore, 2=-2x

Re: Each of 90 students participated at least 1 of the track try [#permalink]

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22 Jan 2016, 20:49

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

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