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Intern  Joined: 18 Jan 2012
Posts: 21
If r and s are integers, is r+s divisible by 3?  [#permalink]

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Difficulty:   15% (low)

Question Stats: 74% (00:50) correct 26% (00:53) wrong based on 500 sessions

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If r and s are integers, is r+s divisible by 3?

(1) s is divisible by 3
(2) r is not divisible by 3

Originally posted by nimc2012 on 07 Feb 2012, 20:43.
Last edited by Bunuel on 08 Feb 2012, 02:11, edited 1 time in total.
Edited the question
Math Expert V
Joined: 02 Sep 2009
Posts: 58402
Re: Number Properties - Question 3  [#permalink]

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8
If r and s are integers, is r+s divisible by 3?

(1) s is divisible by 3 --> s={a multiple of 3}. Not sufficient as no info about r.
(2) r is not divisible by 3 --> r={NOT a multiple of 3}. Not sufficient as no info about s.

(1)+(2) r+s={NOT multiple of 3}+{a multiple of 3}={NOT multiple of 3}. Sufficient.

Below might help to understand this concept better.

If integers $$a$$ and $$b$$ are both multiples of some integer $$k>1$$ (divisible by $$k$$), then their sum and difference will also be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=6$$ and $$b=9$$, both divisible by 3 ---> $$a+b=15$$ and $$a-b=-3$$, again both divisible by 3.

If out of integers $$a$$ and $$b$$ one is a multiple of some integer $$k>1$$ and another is not, then their sum and difference will NOT be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=6$$, divisible by 3 and $$b=5$$, not divisible by 3 ---> $$a+b=11$$ and $$a-b=1$$, neither is divisible by 3.

If integers $$a$$ and $$b$$ both are NOT multiples of some integer $$k>1$$ (divisible by $$k$$), then their sum and difference may or may not be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=5$$ and $$b=4$$, neither is divisible by 3 ---> $$a+b=9$$, is divisible by 3 and $$a-b=1$$, is not divisible by 3;
OR: $$a=6$$ and $$b=3$$, neither is divisible by 5 ---> $$a+b=9$$ and $$a-b=3$$, neither is divisible by 5;
OR: $$a=2$$ and $$b=2$$, neither is divisible by 4 ---> $$a+b=4$$ and $$a-b=0$$, both are divisible by 4.

Hope it's clear.
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General Discussion
Magoosh GMAT Instructor G
Joined: 28 Dec 2011
Posts: 4472
Re: Number Properties - Question 3  [#permalink]

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Hi, there. I'm happy to help with this --- it looks like a cool question. Prompt: if r and s are integers, is r+s divisible by 3?

There are two ways to add up two integers and get a sum that's divisible by 3.

(a) both r and s individually are multiples of 3, so their sum will be a multiple of 3. (Example: 9 + 6 = 15)

(b) let's say that neither r nor s is divisible by 3; let's say that when we divide r by 3, the remainder is m, and when we divide s by 3, the remainder is n; if m + n = 3, then r + s will be a multiple of 3.
Example:
When 5 is divided by 3, remainder = 2
When 7 is divided by 3, remainder = 1
2+1 = 3 ----> 5 + 7 = 12

The basic idea there is that the sum of the remainders will be the remainder of the sum. If sum of the remainders is divisible by 3, then when the sum of the numbers is divided by three, it will go into the sum of the remainders evenly, and therefore go into the entire sum evenly.

BTW, in everything I've said between the prompt and this line, you could replace 3 with any other positive integer greater than 1, and it would still be true.

If we're in one of these two possibilities, then we will know that r+s is divisible by 3.

Statement #1: s is divisible by 3

Promising, but we know nothing about r, so this statement, by itself, is insufficient.

Statement #2: r is not divisible by 3

Again, this could be a step in the right direction, but now we know nothing about s, so this statement, by itself, is insufficient.

Combined Statements #1 & #2: s is divisible by 3 and r is not divisible by 3

Well, neither of the scenarios above allow for one number divisible by 3 and the other not divisible by 3. In fact, when we divide (r+s) by 3, we know the s part will not have a remainder but the r part will, which means that 3 does not go evenly into the sum r+s. Therefore, we can give a definitive "no" answer to the prompt question, and because we have the ability to give a definitive answer, that means we must have sufficient information.

Statements are insufficient individually but are sufficient combined ==> Answer = C.

Does all that make sense?

Here's a particularly difficult divisibility DS question, for more practice.

http://gmat.magoosh.com/questions/871

Let me know if you have any questions about anything I have said here.

Mike _________________
Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)
Intern  Joined: 10 Jan 2012
Posts: 4
Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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I thought the answer will be E because r could be negative 3, and s can be 3. As a result we get -3+3/3= which is not divisible by 3?
I'm confused. Can someone help me to explain it?
Math Expert V
Joined: 02 Sep 2009
Posts: 58402
Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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nobelgirl777 wrote:
I thought the answer will be E because r could be negative 3, and s can be 3. As a result we get -3+3/3= which is not divisible by 3?
I'm confused. Can someone help me to explain it?

Welcome to GMAT Club. Below is an answer to your question.

The red parts in your post are not correct.

1. r cannot be -3, since we are told that "r is NOT divisible by 3", while -3 is clearly divisible by 3. Note that integer $$a$$ is divisible by integer $$b$$ (integer $$a$$ is a multiple of integer $$b$$) means that $$\frac{a}{b}=integer$$, so as -3/3=-1=integer then -3 is divisible by 3.

2. Also if r=-3 (if r could be -3) and s=3 then r+s=-3+3=0 and zero is divisible by every integer except zero itself.

Hope it's clear.
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Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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Got it! Thank you very much!
Director  S
Joined: 12 Nov 2016
Posts: 700
Location: United States
Schools: Yale '18
GMAT 1: 650 Q43 V37 GRE 1: Q157 V158 GPA: 2.66
Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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Bunuel wrote:
If r and s are integers, is r+s divisible by 3?

(1) s is divisible by 3 --> s={a multiple of 3}. Not sufficient as no info about r.
(2) r is not divisible by 3 --> r={NOT a multiple of 3}. Not sufficient as no info about s.

(1)+(2) r+s={NOT multiple of 3}+{a multiple of 3}={NOT multiple of 3}. Sufficient.

Below might help to understand this concept better.

If integers $$a$$ and $$b$$ are both multiples of some integer $$k>1$$ (divisible by $$k$$), then their sum and difference will also be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=6$$ and $$b=9$$, both divisible by 3 ---> $$a+b=15$$ and $$a-b=-3$$, again both divisible by 3.

If out of integers $$a$$ and $$b$$ one is a multiple of some integer $$k>1$$ and another is not, then their sum and difference will NOT be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=6$$, divisible by 3 and $$b=5$$, not divisible by 3 ---> $$a+b=11$$ and $$a-b=1$$, neither is divisible by 3.

If integers $$a$$ and $$b$$ both are NOT multiples of some integer $$k>1$$ (divisible by $$k$$), then their sum and difference may or may not be a multiple of $$k$$ (divisible by $$k$$):
Example: $$a=5$$ and $$b=4$$, neither is divisible by 3 ---> $$a+b=9$$, is divisible by 3 and $$a-b=1$$, is not divisible by 3;
OR: $$a=6$$ and $$b=3$$, neither is divisible by 5 ---> $$a+b=9$$ and $$a-b=3$$, neither is divisible by 5;
OR: $$a=2$$ and $$b=2$$, neither is divisible by 4 ---> $$a+b=4$$ and $$a-b=0$$, both are divisible by 4.

Hope it's clear.

Seriously couldn't have said it better
Director  S
Joined: 12 Nov 2016
Posts: 700
Location: United States
Schools: Yale '18
GMAT 1: 650 Q43 V37 GRE 1: Q157 V158 GPA: 2.66
Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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nimc2012 wrote:
If r and s are integers, is r+s divisible by 3?

(1) s is divisible by 3
(2) r is not divisible by 3

The concept in this question is basically testing three fundamental scenarios

Scenario 1

X + Y / some multiple K- in order for the sum of X and Y to be a multiple of K both X and Y must be a multiple of K

Scenario 2

X + Y/ some multiple K- if either X or Y is not a multiple of K and the other is ...then the sum of X and Y cannot be a multiple of k

Scenario 2

X +Y/ some multiple K - Actually, the sum could be a multiple of K but not doesn't necessarily have to

C
Non-Human User Joined: 09 Sep 2013
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Re: If r and s are integers, is r+s divisible by 3?  [#permalink]

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_________________ Re: If r and s are integers, is r+s divisible by 3?   [#permalink] 01 Oct 2018, 22:57
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