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If a, b, and c are positive integers, what is the remainder

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If a, b, and c are positive integers, what is the remainder [#permalink] New post 03 Feb 2012, 20:45
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If a, b, and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3
(2) b = (c + 1)^3
[Reveal] Spoiler: OA

Last edited by Bunuel on 25 Jun 2014, 00:59, edited 2 times in total.
Edited the question and added the OA
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Re: If a, b, and c are positive integers, what is the remainder [#permalink] New post 04 Feb 2012, 02:55
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Smita04 wrote:
If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3
(2) b = (c + 1)^3


If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3 --> no info about b. Not sufficient.
(2) b = (c + 1)^3 --> no info about b. Not sufficient.

When taken together you can go with algebraic approach or plug-in method:

Algebraic approach:

(1)+(2) Important tip: x^3-y^3 can be factored as follows: x^3-y^3=(x-y)(x^2+xy+y^2). Apply this factoring to b-a --> b-a=(c + 1)^3-c^3=(c+1-c)(c^2+2c+1+c^2+c+c^2)=3c^2+3c+1=3(c^2+c)+1 --> remainder upon division this expression by 3 is 1. Sufficient.

Answer: C.

Plug-in method approach:

(1)+(2) try some numbers for a and b:
a=c^3=1 --> b=(c+1)^3=8 and --> b-a=(c + 1)^3-c^3=7 --> remainder upon division 3 is 1;
a=c^3=8 --> b=(c+1)^3=27 and --> b-a=(c + 1)^3-c^3=19 --> remainder upon division 3 is 1;
a=c^3=27 --> b=(c+1)^3=64 and --> b-a=(c + 1)^3-c^3=37 --> remainder upon division 3 is 1;
a=c^3=64 --> b=(c+1)^3=125 and --> b-a=(c + 1)^3-c^3=61 --> remainder upon division 3 is 1;
...
It seem that there is some kind of pattern and we can safely assume that in all other cases remainder will also be 1. Sufficient.

Answer: C.

Hope it helps.
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Re: If a, b, and c are positive integers, what is the remainder [#permalink] New post 10 Oct 2012, 14:07
(A) & (B) ruled out as we do not get complete info about a & b together, because in question we are supposed to find
remainder when (b-a) is divided by 3.

lets take both together;

Given:
a=c^3;b=(c+1)^3 ) ( for a,b,c all positive integer)
so lets check at three points;
@c=1, a=1,b=8 ; @ c=2, a=8, b= 27, @ c= 3 , a=27,b=64

8-1/3 R->1
27-8/3 R->1
64-27/3 R->1

Hence C is the answer
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Re: If a, b, and c are positive integers, what is the remainder [#permalink] New post 10 Oct 2012, 15:01
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Bunuel wrote:
Smita04 wrote:
If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3
(2) b = (c + 1)^3


If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3 --> no info about b. Not sufficient.
(2) b = (c + 1)^3 --> no info about b. Not sufficient.

When taken together you can go with algebraic approach or plug-in method:

Algebraic approach:

(1)+(2) Important tip: x^3-y^3 can be factored as follows: x^3-y^3=(x-y)(x^2+xy+y^2). Apply this factoring to b-a --> b-a=(c + 1)^3-c^3=(c+1-c)(c^2+2c+1+c^2+c+c^2)=3c^2+3c+1=3(c^2+c)+1 --> remainder upon division this expression by 3 is 1. Sufficient.

Answer: C.

Plug-in method approach:

(1)+(2) try some numbers for a and b:
a=c^3=1 --> b=(c+1)^3=8 and --> b-a=(c + 1)^3-c^3=7 --> remainder upon division 3 is 1;
a=c^3=8 --> b=(c+1)^3=27 and --> b-a=(c + 1)^3-c^3=19 --> remainder upon division 3 is 1;
a=c^3=27 --> b=(c+1)^3=64 and --> b-a=(c + 1)^3-c^3=37 --> remainder upon division 3 is 1;
a=c^3=64 --> b=(c+1)^3=125 and --> b-a=(c + 1)^3-c^3=61 --> remainder upon division 3 is 1;
...
It seem that there is some kind of pattern and we can safely assume that in all other cases remainder will also be 1. Sufficient.

Answer: C.

Hope it helps.



Integers, when divided by 3 can leave a remainder of 0, 1, or 2.
Integers cubed, when divided by 3, will leave the same remainders, because 0^3=0, 1^3=1, and 2^3=8=6+2.

Therefore, when subtracting cubes of two consecutive integers, the result will always leave a remainder of 1:
the remainders repeat themselves cyclically 0,1,2,0,1,2,..., so 1-0=2-1=1 and \,\,0-2=-3+1.
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Re: If a, b, and c are positive integers, what is the remainder [#permalink] New post 24 Jun 2014, 23:52
Bunuel wrote:
Smita04 wrote:
If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3
(2) b = (c + 1)^3


If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3 --> no info about b. Not sufficient.
(2) b = (c + 1)^3 --> no info about b. Not sufficient.

When taken together you can go with algebraic approach or plug-in method:

Algebraic approach:

(1)+(2) Important tip: x^3-y^3 can be factored as follows: x^3-y^3=(x-y)(x^2+xy+y^2). Apply this factoring to b-a --> b-a=(c + 1)^3-c^3=(c+1-c)(c^2+2c+1+c^2+c+c^2)=3c^2+3c+1=3(c^2+c)+1 --> remainder upon division this expression by 3 is 1. Sufficient.

Answer: C.

Plug-in method approach:

(1)+(2) try some numbers for a and b:
a=c^3=1 --> b=(c+1)^3=8 and --> b-a=(c + 1)^3-c^3=7 --> remainder upon division 3 is 1;
a=c^3=8 --> b=(c+1)^3=27 and --> b-a=(c + 1)^3-c^3=19 --> remainder upon division 3 is 1;
a=c^3=27 --> b=(c+1)^3=64 and --> b-a=(c + 1)^3-c^3=37 --> remainder upon division 3 is 1;
a=c^3=64 --> b=(c+1)^3=125 and --> b-a=(c + 1)^3-c^3=61 --> remainder upon division 3 is 1;
...
It seem that there is some kind of pattern and we can safely assume that in all other cases remainder will also be 1. Sufficient.

Answer: C.

Hope it helps.


Hi Bunuel,
Would it be possible for you to provide OA to the question and adjust the difficulty level, so that we guys can actually have some appreciation for the question?
:)

Thanks!
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Re: If a, b, and c are positive integers, what is the remainder [#permalink] New post 25 Jun 2014, 01:00
1
This post received
KUDOS
Expert's post
neo656 wrote:
Bunuel wrote:
Smita04 wrote:
If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3
(2) b = (c + 1)^3


If a , b , and c are positive integers, what is the remainder after b - a is divided by 3?

(1) a = c^3 --> no info about b. Not sufficient.
(2) b = (c + 1)^3 --> no info about b. Not sufficient.

When taken together you can go with algebraic approach or plug-in method:

Algebraic approach:

(1)+(2) Important tip: x^3-y^3 can be factored as follows: x^3-y^3=(x-y)(x^2+xy+y^2). Apply this factoring to b-a --> b-a=(c + 1)^3-c^3=(c+1-c)(c^2+2c+1+c^2+c+c^2)=3c^2+3c+1=3(c^2+c)+1 --> remainder upon division this expression by 3 is 1. Sufficient.

Answer: C.

Plug-in method approach:

(1)+(2) try some numbers for a and b:
a=c^3=1 --> b=(c+1)^3=8 and --> b-a=(c + 1)^3-c^3=7 --> remainder upon division 3 is 1;
a=c^3=8 --> b=(c+1)^3=27 and --> b-a=(c + 1)^3-c^3=19 --> remainder upon division 3 is 1;
a=c^3=27 --> b=(c+1)^3=64 and --> b-a=(c + 1)^3-c^3=37 --> remainder upon division 3 is 1;
a=c^3=64 --> b=(c+1)^3=125 and --> b-a=(c + 1)^3-c^3=61 --> remainder upon division 3 is 1;
...
It seem that there is some kind of pattern and we can safely assume that in all other cases remainder will also be 1. Sufficient.

Answer: C.

Hope it helps.


Hi Bunuel,
Would it be possible for you to provide OA to the question and adjust the difficulty level, so that we guys can actually have some appreciation for the question?
:)

Thanks!


Added the OA. Thank you.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If a, b, and c are positive integers, what is the remainder   [#permalink] 25 Jun 2014, 01:00
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