November 22, 2018 November 22, 2018 10:00 PM PST 11:00 PM PST Mark your calendars  All GMAT Club Tests are free and open November 22nd to celebrate Thanksgiving Day! Access will be available from 0:01 AM to 11:59 PM, Pacific Time (USA) November 23, 2018 November 23, 2018 10:00 PM PST 11:00 PM PST Practice the one most important Quant section  Integer properties, and rapidly improve your skills.
Author 
Message 
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 50711

Number Properties: Tips and hints
[#permalink]
Show Tags
04 Jun 2014, 11:25
Number Properties: Tips and hints EVEN/ODD1. An even number is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without a remainder. 2. An odd number is an integer that is not evenly divisible by 2. 3. According to the above both negative and positive integers can be even or odd. ZERO:1. 0 is an integer. 2. 0 is an even integer. An even number is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without a remainder and as zero is evenly divisible by 2 then it must be even. 3. 0 is neither positive nor negative integer (the only one of this kind). 4. 0 is divisible by EVERY integer except 0 itself, (or, which is the same, zero is a multiple of every integer except zero itself). PRIME NUMBERS:1. 1 is not a prime, since it only has one divisor, namely 1. 2. Only positive numbers can be primes. 3. There are infinitely many prime numbers. 4. the only even prime number is 2. Also 2 is the smallest prime. 5. All prime numbers except 2 and 5 end in 1, 3, 7 or 9. PERFECT SQUARES1. The number of distinct factors of a perfect square is ALWAYS ODD. The reverse is also true: if a number has the odd number of distinct factors then it's a perfect square; 2. The sum of distinct factors of a perfect square is ALWAYS ODD. The reverse is NOT always true: a number may have the odd sum of its distinct factors and not be a perfect square. For example: 2, 8, 18 or 50; 3. A perfect square ALWAYS has an ODD number of Oddfactors, and EVEN number of Evenfactors. The reverse is also true: if a number has an ODD number of Oddfactors, and EVEN number of Evenfactors then it's a perfect square. For example: odd factors of 36 are 1, 3 and 9 (3 odd factor) and even factors are 2, 4, 6, 12, 18 and 36 (6 even factors); 4. Perfect square always has even powers of its prime factors. The reverse is also true: if a number has even powers of its prime factors then it's a perfect square. For example: \(36=2^2*3^2\), powers of prime factors 2 and 3 are even. IRRATIONAL NUMBERS1. An irrational number is any real number that cannot be expressed as a ratio of integers. 2. The square root of any positive integer is either an integer or an irrational number. So, \(\sqrt{x}=\sqrt{integer}\) cannot be a fraction, for example it cannot equal to 1/2, 3/7, 19/2, ... It MUST be an integer (0, 1, 2, 3, ...) or irrational number (for example \(\sqrt{2}\), \(\sqrt{3}\), \(\sqrt{17}\), ...). Please share your number properties tips below and get kudos point. Thank you.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Intern
Status: Into the last good fight
Joined: 22 Feb 2012
Posts: 17
Location: Bangladesh
GMAT 1: 550 Q42 V24 GMAT 2: 550 Q44 V23 GMAT 3: 540 Q41 V24

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
24 Jun 2014, 22:15
[wrapimg=]PERFECT SQUARES 1. The number of distinct factors of a perfect square is ALWAYS ODD. The reverse is also true: if a number has the odd number of distinct factors then it's a perfect square;
3. A perfect square ALWAYS has an ODD number of Oddfactors, and EVEN number of Evenfactors. The reverse is also true: if a number has an ODD number of Oddfactors, and EVEN number of Evenfactors then it's a perfect square. For example: odd factors of 36 are 1, 3 and 9 (3 odd factor) and even factors are 2, 4, 6, 12, 18 and 36 (6 even factors);[/wrapimg]
I was learning the above points, while a confusion rose. I'd be grateful if you could explain.
We know that 100 is a perfect square (10^2) If we Prime factorize 100, we get
100= 2*2*5*5
number 1 tips says that a perfect square has odd number of distinct factors. Here 100 has two distinct factors (2,5). How that can be explained ?
number 3 tips says that a perfect square always has an odd number of odd factors and even number of even factors. Here we see, 100 has two 5's which is even number.
Even if we see number 3 tips as, a perfect square always has an odd number of DISTINCT odd factors and even number of DISTINCT even factors., we find that, 100 has only ONE DISTINCT even factor which is 2
Please explain.



Math Expert
Joined: 02 Sep 2009
Posts: 50711

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
25 Jun 2014, 01:42
Tanvr wrote: [wrapimg=]PERFECT SQUARES 1. The number of distinct factors of a perfect square is ALWAYS ODD. The reverse is also true: if a number has the odd number of distinct factors then it's a perfect square;
3. A perfect square ALWAYS has an ODD number of Oddfactors, and EVEN number of Evenfactors. The reverse is also true: if a number has an ODD number of Oddfactors, and EVEN number of Evenfactors then it's a perfect square. For example: odd factors of 36 are 1, 3 and 9 (3 odd factor) and even factors are 2, 4, 6, 12, 18 and 36 (6 even factors);[/wrapimg]
I was learning the above points, while a confusion rose. I'd be grateful if you could explain.
We know that 100 is a perfect square (10^2) If we Prime factorize 100, we get
100= 2*2*5*5
number 1 tips says that a perfect square has odd number of distinct factors. Here 100 has two distinct factors (2,5). How that can be explained ?
number 3 tips says that a perfect square always has an odd number of odd factors and even number of even factors. Here we see, 100 has two 5's which is even number.
Even if we see number 3 tips as, a perfect square always has an odd number of DISTINCT odd factors and even number of DISTINCT even factors., we find that, 100 has only ONE DISTINCT even factor which is 2
Please explain. 2 and 5 are prime factors of 100. The total number of factors of 100=2^2*5^2 is (2+1)(2+1)=9=odd: 1, 2, 4, 5, 10, 20, 25, 50, 100. Out of these 9 factors three are odd (1, 5, and 25) and 6 are even (2, 4, 10, 20, 50, 100). Hope it helps.
_________________
New to the Math Forum? Please read this: Ultimate GMAT Quantitative Megathread  All You Need for Quant  PLEASE READ AND FOLLOW: 12 Rules for Posting!!! Resources: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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.
What are GMAT Club Tests? Extrahard Quant Tests with Brilliant Analytics



Intern
Joined: 07 Jul 2014
Posts: 16

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
23 Jul 2014, 11:29
Can I suggest a few new topics? Specifically Geometry, word problems, and combinatrics/probability?



Math Expert
Joined: 02 Sep 2009
Posts: 50711

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
23 Jul 2014, 11:51



Intern
Joined: 27 Aug 2014
Posts: 27
GMAT Date: 09272014

Number Properties: Tips and hints
[#permalink]
Show Tags
27 Aug 2014, 05:37
If progressions comes under this topic, would like to add this tip.
In the specific case of sum to n1 terms being equal to sum to n2 terms of the same arithmetic progression, the sum of the term numbers which exhibit equal sums is constant for the given evenly spaced set of numbers.
(S3 denotes Sum of the first three terms of the evenly spaced set.)
1. Q: if sum to 11 terms equal sum to 19 terms in an evenly spaced set, what is the sum to 30 terms for this series? A: S11 = S19; so S0 = S30. Since S0 = 0, S30 = 0.
2. This happens because the arithmetic progression's negative terms cancel out the positive terms.
Also, if the series has a zero in it, the sum will be equal for two terms such that one term number will be odd and the other will be even. Ex.: 10, 5, 0, 5, 10.....
And if the series does not have a zero in it, the sum will be equal for two terms such that both term numbers will be either odd or even.
Ex.: 12, 4, 4, 12......



Intern
Joined: 27 Aug 2014
Posts: 27
GMAT Date: 09272014

Number Properties: Tips and hints
[#permalink]
Show Tags
27 Aug 2014, 05:52
Another tip,
If B is 1/x times more than A, then A is 1/(x+1) times lesser than B.
This is especially useful in averages, profit and loss, time rate questions.
Example:
If B's wage is 25% more than A's wage, then what is A's wage in terms of B?
B is 1/4 times more than A, so A will be 1/5 or 20% lesser than B. i.e., A = 80% of B



Intern
Joined: 27 Aug 2014
Posts: 27
GMAT Date: 09272014

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
27 Aug 2014, 07:15
Tip for questions involving recurring decimals:
Note the following pattern for repeating decimals: 0.22222222... = 2/9 0.54545454... = 54/99 0.298298298... = 298/999
Note the pattern if zeroes preceed the repeating decimal: 0.022222222... = 2/90 0.00054545454... = 54/99000 0.00298298298... = 298/99900



Director
Joined: 02 Sep 2016
Posts: 688

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
12 Jan 2017, 09:41
Where can I access the file for geometry and word problems?
_________________
Help me make my explanation better by providing a logical feedback.
If you liked the post, HIT KUDOS !!
Don't quit.............Do it.



Math Expert
Joined: 02 Sep 2009
Posts: 50711

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
12 Jan 2017, 23:21



Director
Joined: 02 Sep 2016
Posts: 688

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
13 Jan 2017, 02:39
Bunuel wrote: Shiv2016 wrote: Where can I access the file for geometry and word problems? Go through the following topic ALL YOU NEED FOR QUANT ! ! !. Hope it helps. Great compilation. Thanks Bunuel.



Intern
Joined: 29 Aug 2016
Posts: 21
GPA: 3

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
19 Jan 2017, 08:05
a very helpful post...



Manager
Joined: 20 Feb 2017
Posts: 73

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
27 Aug 2017, 09:56
Hi Bunuel,
I have a query regarding the prime numbers. In DS question if it is told that numbers are consecutive prime numbers then from where we have to start numbers?
Thanks in advance.



NonHuman User
Joined: 09 Sep 2013
Posts: 8863

Re: Number Properties: Tips and hints
[#permalink]
Show Tags
29 Aug 2018, 07:13
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). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources




Re: Number Properties: Tips and hints &nbs
[#permalink]
29 Aug 2018, 07:13






