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Maths, Physics and chemistry books are stored on a library [#permalink]
26 May 2008, 11:01

1

This post was BOOKMARKED

00:00

A

B

C

D

E

Difficulty:

35% (medium)

Question Stats:

72% (04:39) correct
28% (02:56) wrong based on 69 sessions

Maths, Physics and chemistry books are stored on a library shelf that can accommodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the probability, that a book selected at random is either a hard cover book or a chemistry book?

Maths, Physics and chemistry books are stored on a library shelf that can accomodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 4 hard-cover books among the maths and physics books. What is the prob, that a book selected at random is either a hard cover book or a chemistry book?

1/10 3/20 1/5 1/4 9/20

the shelf has the capacity of 25 books, 20% empty (5 books), the shelf has 20 books

So the shelf only has 12 math, 6 phys, 2 chem books

If there are 12 soft covers and 8 hard covers, and 4 hard covers are math/phys, then 4 hard cover must be chemistry, BUT there are only 2 chemistry books.

Maths, Physics and chemistry books are stored on a library shelf that can accomodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the prob, that a book selected at random is either a hard cover book or a chemistry book?

Chemistry books: 2 Physics books: 6 Math books: 12

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
30 Mar 2014, 09:32

1

This post received KUDOS

First phase of this problem requires you to determine how many mathematics and chemistry books are even on the shelf. To do so, you have the equations:

m + p + c = 20 (since 4/5 of the 25 spots are full of books)

m = 2p

p = 4 + c

From that, you can use Substitution to get everything down to one variable.

c = p - 4

m = 2p

p = p

Then (p - 4) + 2p + p = 20, so 4p = 24 and p = 6. That means that there are 12 math, 6 physics, and 2 chemistry books on the shelf.

With those numbers, you also know that there are 8 total hardcovers, 1 of which is chemistry. So if your goal is to get either a hardcover or a chemistry, there are 9 ways to "win" - either one of the 7 hardcovers that aren't chemistry or the two chemistry books. So out of the 20 total, 9 provide the desired outcome, making the answer E.

Note - a common trap answer here is A, as people forget that "2 chem + 8 hard" double counts the one book that is "both". Venn Diagram logic can be helpful to avoid that trap, or in cases like this with relatively small numbers it may be even more convenient to just write out the possibilities.

Maths, Physics and chemistry books are stored on a library shelf that can accomodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the prob, that a book selected at random is either a hard cover book or a chemistry book?

Chemistry books: 2 Physics books: 6 Math books: 12

P(AuB) - P(AnB) = 21/50 (since either means that we don't want both)

What is wrong here? (OA: 9/20)

Everything is correct except this: P(AnB) = P(A)*(B) = (1/10)*(2/5) = 2/50 P(AnB) is given by the product of individual probabilities when the events are independent. How many Chemistry books are hardcover? There are total 8 hardcovers and 7 belong to Math and Physics. This means there is only 1 Chemistry hardcover book out of total 20 books. So P(AnB) = 1/20

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
01 Apr 2014, 22:12

@GDR29,

Thanks for the explanation but I have one doubt.

We know that there are two chemistry books and there is no information like atleast each of Maths, Physics and Checmistry has to be either soft or hard covered. In such case, there will be two chemistry books which may be hard covered.

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
02 Apr 2014, 06:22

Hi guys, After knowing there are 20 books on the shelve, I've just used the "Among all the books, 12 books are soft cover and the remaining are hard-cover" info. Using this info you get that 8/20 books are hard cover... and at least one book has to be chemestry, so the chances of getting either a hard cover book or a chemestry one have to be higher than 8/20 --- only alternative is E) 9/20 What do you think? Thanks!

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
02 Apr 2014, 21:09

Expert's post

nishant.1987 wrote:

@GDR29,

Thanks for the explanation but I have one doubt.

We know that there are two chemistry books and there is no information like atleast each of Maths, Physics and Checmistry has to be either soft or hard covered. In such case, there will be two chemistry books which may be hard covered.

Please advise.

There is a typo in the question given. You are given that there are 12 softcover books (which means there are 8 hard cover books). You also given that 7 of the hardcover books are either Math or Physics. So there is only 1 Chemistry hard cover book. _________________

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
02 Apr 2014, 21:12

Expert's post

easycheesy wrote:

Hi guys, After knowing there are 20 books on the shelve, I've just used the "Among all the books, 12 books are soft cover and the remaining are hard-cover" info. Using this info you get that 8/20 books are hard cover... and at least one book has to be chemestry, so the chances of getting either a hard cover book or a chemestry one have to be higher than 8/20 --- only alternative is E) 9/20 What do you think? Thanks!

That's correct. Note that we do analyze the rest of the data to say that the required probability has to be greater than 8/20 since it is possible that all Chem books are hardcover and hence already a part of the 8 books. But, to say that the required probability is "greater than or equal to" 8/20, all we need is "Among all the books, 12 books are soft cover and the remaining are hard-cover".

There is only one option greater than or equal to 8/20. _________________

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
11 May 2014, 23:29

1

This post was BOOKMARKED

alimad wrote:

Maths, Physics and chemistry books are stored on a library shelf that can accommodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the probability, that a book selected at random is either a hard cover book or a chemistry book?

8 + 2c + 4 + c + c = 20 4c + 12 = 20 4c = 8 c = 2 , P = 6, M = 12

Please assist further. Thanks

Hello I am not good at probability, but somehow got this answer right . Could you please explain if the approach used by me is correct or it was just a stroke of luck that i got it right..

Number of Math+Physics+Chemistry books= 25 - ((20/100)*25) = 20 Hence, since M=2P, P=4+C, (4+C)+C+2(4+C)=20 Therefore, M=12, C=2, P=6

Probability of getting a chemistry book = 2/20 = 1/10 Probability of getting a Hardcover book = P(getting a math or phy book) * P( getting a hardcover book from math or phy books) = (18/20 )*(7/18) = 7/20

Therefore, P(Getting chemistry or hardcover book) = (1/10)+(7/20) = 9/20

I am really doubtful about the red colored approach. Please guide.

Re: Maths, Physics and chemistry books are stored on a library [#permalink]
12 May 2014, 02:32

Expert's post

khushboobhatia wrote:

alimad wrote:

Maths, Physics and chemistry books are stored on a library shelf that can accommodate 25 books. Currently, 20% of the shelf spots remain empty. There are twice as many maths books as physics books and the number of physics books is 4 greater than that of chemistry books. Among all the books, 12 books are soft cover and the remaining are hard-cover. If there are a total of 7 hard-cover books among the maths and physics books. What is the probability, that a book selected at random is either a hard cover book or a chemistry book?

8 + 2c + 4 + c + c = 20 4c + 12 = 20 4c = 8 c = 2 , P = 6, M = 12

Please assist further. Thanks

Hello I am not good at probability, but somehow got this answer right . Could you please explain if the approach used by me is correct or it was just a stroke of luck that i got it right..

Number of Math+Physics+Chemistry books= 25 - ((20/100)*25) = 20 Hence, since M=2P, P=4+C, (4+C)+C+2(4+C)=20 Therefore, M=12, C=2, P=6

Probability of getting a chemistry book = 2/20 = 1/10 Probability of getting a Hardcover book = P(getting a math or phy book) * P( getting a hardcover book from math or phy books) = (18/20 )*(7/18) = 7/20

Therefore, P(Getting chemistry or hardcover book) = (1/10)+(7/20) = 9/20

I am really doubtful about the red colored approach. Please guide.

Your approach is correct. Though the red part should be {the probability of getting a hardcover math or physics book}. _________________