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If x and yare positive integers , what is the remainder when

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If x and yare positive integers , what is the remainder when [#permalink] New post 03 Jan 2014, 05:12
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The Official Guide For GMAT® Quantitative Review, 2ND Edition

If x and y are positive integers, what is the remainder when 10^x + y is divided by 3 ?

(1) x = 5
(2) y = 2

Data Sufficiency
Question: 16
Category: Arithmetic Properties of numbers
Page: 154
Difficulty: 600


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Re: If x and yare positive integers , what is the remainder when [#permalink] New post 03 Jan 2014, 05:12
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SOLUTION

If x and y are positive integers, what is the remainder when 10^x +y is divided by 3?

Since, the sum of the digits of 10^x is always 1 then the remainders when 10^x+y is divided by 3 is only dependant on the value of the number added to 10^x, so on y. If y is a multiple of 3 then 10^x+y will yield the remainder of 1 (since the sum of the digits of 10^x+y will be one more than a multiple of 3), if y is one more than a multiple of 3 then 10^x+y will yield the remainder of 2 and finally if y is two more than a multiple of 3 then 10^x+y will yield the remainder of 0,

(1) x = 5. Not sufficient.
(2) y = 2. Sufficient.

Answer: B.
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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!!!;

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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 x and yare positive integers , what is the remainder when [#permalink] New post 04 Jan 2014, 01:05
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Bunuel wrote:
The Official Guide For GMAT® Quantitative Review, 2ND Edition

If x and y are positive integers, what is the remainder when 10^x + y is divided by 3 ?

(1) x = 5
(2) y = 2



We can see that 10^x + y will always be of the form 10+y , or 100 +y or 1000+y etc depending on the power of x
so 1 is always going to be carried forward from the 10^x side to y

e.g.

if x= 1 and y = 1 then 10^x+y will give 10+1 =11, divided by 3 remainder 2
if x= 2 and y = 1 then we have 100+1 =101,divided by 3 remainder 2
if x=2 and y =2 then we have 100+2 =102,divided by 3 remainder 0
etc

hence the remainder really depends on the value of y

1) if y = 1 remainder is 2, if y is 2 remainder is 0, since no info about y hence insuff.

2)X=2 , just what we were looking for , no matter what ever is the value of x remainder will always be 0.
as 10^x+y will always be of the form:

12
102
1002
10002
etc remainder in all cases is 0 hence B is suff
answer is B
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Re: If x and yare positive integers , what is the remainder when [#permalink] New post 05 Jan 2014, 10:46
Bunuel wrote:
The Official Guide For GMAT® Quantitative Review, 2ND Edition

If x and y are positive integers, what is the remainder when 10^x + y is divided by 3 ?

(1) x = 5
(2) y = 2



Before diving into the statements, lets try to simplify the expression. (10^x + y) rem by 3 = (1^x + y) rem by 3 = y rem by 3. The rule of simplification is applicable only in case of remainders.

So clearly Statement is Sufficient. hence Option B)
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Re: If x and yare positive integers , what is the remainder when [#permalink] New post 05 Jan 2014, 10:59
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SOLUTION

If x and y are positive integers, what is the remainder when 10^x +y is divided by 3?

Since, the sum of the digits of 10^x is always 1 then the remainders when 10^x+y is divided by 3 is only dependant on the value of the number added to 10^x, so on y. If y is a multiple of 3 then 10^x+y will yield the remainder of 1 (since the sum of the digits of 10^x+y will be one more than a multiple of 3), if y is one more than a multiple of 3 then 10^x+y will yield the remainder of 2 and finally if y is two more than a multiple of 3 then 10^x+y will yield the remainder of 0,

(1) x = 5. Not sufficient.
(2) y = 2. Sufficient.

Answer: B.
_________________

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 x and yare positive integers , what is the remainder when   [#permalink] 05 Jan 2014, 10:59
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