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Of the science books in a certain supply room, 50 are on [#permalink]

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21 Apr 2012, 20:17

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Of the science books in a certain supply room, 50 are on botany, 65 are on zoology, 90 are on physics, 50 are on geology, and 110 are on chemistry. If science books are removed randomly from the supply room, how many must be removed to ensure that 80 of the books removed are on the same science?

16. Of the science books in a certain supply room, 50 are on botany, 65 are on zoology, 90 are on physics, 50 are on geology, and 110 are on chemistry. If science books are removed randomly from the supply room, how many must be removed to ensure that 80 of the books removed are on the same science?

(A) 81 (B) 159 (C) 166 (D) 285 (E) 324

We can remove 80 books only from physics or from chemistry. Worst case scenario would be if we remove all books on botany, zoology, geology, 79 books on physics and 79 books on chemistry.

We would have 50(botany) + 65(zoology) + 50(geology) + 79(physics) + 79(chemistry) = 323 books and still won't have 80 books from the same science. As after this, only books on physics and chemistry will be left then any next book would become the 80th book either on physics or on chemistry.

Pretty much yes. For such kind of questions you should test worst case scenario to get the answer. Check the links in my previous post for similar questions.
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Of the science books in a certain supply room, 50 are on botany, 65 are on zoology, 90 are on physics. 50 are on geology, and 110 are on chemistry. If science books are removed randomly from the supply room, how many must be removed to ensure that 80 of the books removed are on the same science?

Re: Of the science books in a certain supply room, 50 are on [#permalink]

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03 Jan 2016, 11:44

What is the number at which picking one more book will guarantee that a thresold of 80 has been surpassed in one science?

Imagine you are playing a rigged game. The object of the game is for you to draw 80 books from the same science. The person attempting to cheat you out of winning picks out the books for you. He or she tries to prevent you from drawing the winning number of books until it is no longer possible to do so. He or she first starts off by picking books that cannot reach the 80 threshold and does so until such books are exhausted. He or she then starts picking books out of one the two remaining sciences until he reaches 79. To delay reaching the 80 thresold as much as possible he or she then shifts to the last science until he or she reaches 79. At this point it is no longer possible to cheat you out of winning and when the next book is drawn, you meet the 80 book threshold. The number of books drawn at that point is therefore 50 + 65 + 50 + 79 + 79 + 1 = 324.

Re: Of the science books in a certain supply room, 50 are on [#permalink]

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09 May 2016, 15:02

I had a different approach.

You know that only books you can remove to get 80 of the same science is Physics (90) or Chemistry (110). Probability of drawing out a Chemistry is higher with every draw 110/365 vs 80/365. Logically you'll need to take out fewer books to 80 of the Chemistry. So you look at Physics books instead. Because the question says 'ensure'.

P(Physics) = 90/365 simplify to 90/360 or 1/4. You know that for every 4 books you take out 1 is likely to be a physics book. Then to get 80 physics books you need to take out 80 x 4 = 320. Answer is (E)

Re: Of the science books in a certain supply room, 50 are on [#permalink]

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11 May 2016, 19:31

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[quote="Bunuel"]Bumping for review and further discussion*. Get a kudos point for an alternative solution!

Had an alternative approach

To ensure you get at least 80 books, you only look at Physics or Chemistry. 90 or 110. I calculated what percentage 80 books were of each group and took the highest percentage (i.e. 80/90 for Physics) and multiplied it by the total number of books (365). This makes sure you get 80 of one science as it makes sure you get 80 of the science that has the fewer books (i.e. harder probability). This gives you exactly 324.

Re: Of the science books in a certain supply room, 50 are on [#permalink]

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10 Aug 2017, 09:27

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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