[ 7 posts ]

 Print view Previous topic | Next topic

#### Divisibility & Primes, Ch 1, Q5

Author Message
 Post subject: Divisibility & Primes, Ch 1, Q5  Posted: Wed Mar 02, 2011 5:39 am
 Students

Posts: 2
 I have a question about the very first chapter of Divisibility & Primes. Question 5 states that j is divisible by 12 and 10 and asks the question whether it is divisible by 24.What I do is that I prime factorise 12 and 10 and put all the prime factors in a partial prime box. This is what I have been taught to do in the chapter. Since I can build factors out of the prime factors, I multiply 2*2*2*3=24 and state that 24 is a factor. Of course I see that with a numerical example 24 does not necessarily have to be a factor. But I do not get why one of the 2's may be redundant. From what information in the text am I supposed to know that one 2 may be redundant?

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Thu Mar 03, 2011 5:09 pm
 Course Students

Posts: 4
 you should look at this information: divisible by 12 and 10.one of ways to know what factors are redundant isJ is divisible by 12, so prime factor are 2, 2, 3again J is also divisible 10, so prime factors are 2, 5. in above, first statement has two 2's and 2nd statement has one 2. so you can say one 2 can be redundant.another example: J is divisible by 12 and 18J is divisible by 12, so prime factor are 2, 2, 3again J is also divisible 18, so prime factors are 2,3,3. in above, first statement has two 2's and 2nd statement has one 2. so you can draw one 2 can be redundant. again first statement has one 3 and 2nd statement has two 3's. so you can say one 2 and one 3 are redundant.I hope this helps

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Fri Mar 04, 2011 9:33 pm
 ManhattanGMAT Staff

Posts: 4404
Location: Southwest Airlines, seat 21C
 one thing to keep in mind about prime boxes is that you should never include more in a box than you absolutely need. you find out a number is divisible by 12, so you can put 2 2 3 in the prime box. then you find out it is also divisible by 10, so you ask yourself what MORE you need in the box. you already have a 2, so just put in a 5 and now your prime box has 2 2 3 5 (and possibly more, but these are the numbers you know). this set of numbers can build a 12 and also a 10, but we don't know if we can build a 24.. _________________Tim SandersManhattan GMAT Instructor

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Mon Mar 07, 2011 5:42 pm
 Course Students

Posts: 34
 Somebody else posted this when I asked this question (I hope it's ok I am reposting someone elses answer):Written by:tylrhllnbchPicture a small, one-room building. This building has two windows in it, on different sides of the building. You can't see in either window. Susie goes up to Window 1, looks in, and comes over to tell you that there are a 2, a 2, and a 3 inside that room. (2, 2, and 3 are the prime factors of 12).So what do you know? There are a 2, a 2, and a 3 in that room.Then Amy goes up to Window 2, looks in, and comes over to tell you that there are a 2 and a 5 inside that room (2 and 5 are the prime factors of 10).So what do you know? There are a 2 and a 5 in that room.Now think about what you definitely know based on the info from BOTH Susie and Amy. There's definitely a 3 in the room. There's also a 5. What about the 2's? Are there definitely three 2's in there? Or could Susie and Amy have been looking at the same 2? We know there are at least two 2's in the room, because Susie reported two separate 2's. But that one 2 that Amy saw - that could have been one of the 2's that Susie saw. So I don't know for sure that there is a 3rd 2 in the room - there might be, but I just don't know.What's that amount to? 2, 2, 3, 5 - just like we figured out from trying numbers. In other words, each statement gives us true BUT potentially overlapping information - and we have to strip out the overlap when we combine the statements.

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Tue Mar 08, 2011 1:58 pm
 ManhattanGMAT Staff

Posts: 2390
 That's a great analogy afvatcha, and exactly right. Thanks for posting! _________________Jamie NelsonManhattanGMAT Instructor

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Tue Mar 08, 2011 5:51 pm
 Course Students

Posts: 34
 jnelson0612 wrote:That's a great analogy afvatcha, and exactly right. Thanks for posting!Just to be completely safe...and honest....I DON NOT come up with that one, but thanks!

 Post subject: Re: Divisibility & Primes, Ch 1, Q5  Posted: Tue Mar 15, 2011 3:57 pm
 ManhattanGMAT Staff

Posts: 4404
Location: Southwest Airlines, seat 21C
 :) _________________Tim SandersManhattan GMAT Instructor

Display posts from previous:  Sort by

[ 7 posts ]

#### Who is online

 Users browsing this forum: No registered users and 1 guest

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

Search for:
 Jump to:  Select a forum ------------------ Ask An Instructor    General GMAT Strategy Questions    GMAT Math       General Math Questions       GMAT Prep Math       Manhattan GMAT CAT Math       Manhattan GMAT Non-CAT Math       Official Guide Math    GMAT Verbal       General Verbal Questions       GMAT Prep Verbal       Manhattan GMAT CAT Verbal       Manhattan GMAT Non-CAT Verbal       Official Guide Verbal    GMAT Integrated Reasoning (IR)       GMAT Prep IR       Manhattan GMAT CAT IR       Manhattan GMAT Non-CAT IR    GMAT AWA Essays Manhattan GMAT    Ask Student Services    Study Groups    GMAT Test Day    Test Centers    Instructor Feedback    Course Feedback B-School    Ask an mbaMission Admissions Consultant    B-School Essays    Business Schools    Professional Networking