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Bunuel
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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
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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

On test day people (including myself) might scramble and forget formulas. Would a solution like so work just as well?

B = (5/4)A --> A = (4/5)B

In general, it's much better to understand the reasoning behind these rules than to memorize them. I go into some good ratios reasoning here:
General Discussion
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[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 Odd-factors, and EVEN number of Even-factors. The reverse is also true: if a number has an ODD number of Odd-factors, and EVEN number of Even-factors 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.
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[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 Odd-factors, and EVEN number of Even-factors. The reverse is also true: if a number has an ODD number of Odd-factors, and EVEN number of Even-factors 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.
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Can I suggest a few new topics? Specifically Geometry, word problems, and combinatrics/probability?
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Can I suggest a few new topics? Specifically Geometry, word problems, and combinatrics/probability?

Thank you. Will cover them soon.
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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......
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Where can I access the file for geometry and word problems?
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Where can I access the file for geometry and word problems?

Go through the following topic ALL YOU NEED FOR QUANT ! ! !.

Hope it helps.
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Shiv2016
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.
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a very helpful post...
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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.
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Bakervai
[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 Odd-factors, and EVEN number of Even-factors. The reverse is also true: if a number has an ODD number of Odd-factors, and EVEN number of Even-factors 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.


Were you able to figure it out? I have the same question!!
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Bunuel

Number Properties: Tips and hints


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;
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;


According to (1) and (2)
36 has ODD number of distinct factors...
but discrinct factors of 36 are 2 and 3
i.e. TWO factors i.e. EVEN.
Please explain..

Also, if you are going to include 1 in the list, it makes statement (2) Flawed!!
Because
(A) 1 + 2 + 3 = 6 = Even ≠ Odd


Shouldn't statement (1) use the term Distinct Prime factors?

I'm confused!
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Bakervai
[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 Odd-factors, and EVEN number of Even-factors. The reverse is also true: if a number has an ODD number of Odd-factors, and EVEN number of Even-factors 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.


Were you able to figure it out? I have the same question!!

1. Divisors of 100 are: 1 | 2 | 4 | 5 | 10 | 20 | 25 | 50 | 100 (9 divisors).

2. 100 has 3 odd factors (1, 5, and 25) and 6 even factors (2, 4, 10, 20, 50 and 100)
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Bunuel

Number Properties: Tips and hints


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;
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;


According to (1) and (2)
36 has ODD number of distinct factors...
but discrinct factors of 36 are 2 and 3
i.e. TWO factors i.e. EVEN.
Please explain..

Also, if you are going to include 1 in the list, it makes statement (2) Flawed!!
Because
(A) 1 + 2 + 3 = 6 = Even ≠ Odd


Shouldn't statement (1) use the term Distinct Prime factors?

I'm confused!

Factors of 36 are 1 | 2 | 3 | 4 | 6 | 9 | 12 | 18 | 36 (9 divisors).
Odd factors of 36 are 1, 3, and 9.
Even factors of 36 are 2, 4, 6, 12, 18, and 26.
Prime factors of 36 are 2 and 3: 36 = 2^2*3^2.
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Bunuel
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Bunuel

Number Properties: Tips and hints


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;
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;


According to (1) and (2)
36 has ODD number of distinct factors...
but discrinct factors of 36 are 2 and 3
i.e. TWO factors i.e. EVEN.
Please explain..

Also, if you are going to include 1 in the list, it makes statement (2) Flawed!!
Because
(A) 1 + 2 + 3 = 6 = Even ≠ Odd


Shouldn't statement (1) use the term Distinct Prime factors?

I'm confused!

Factors of 36 are 1 | 2 | 3 | 4 | 6 | 9 | 12 | 18 | 36 (9 divisors).
Odd factors of 36 are 1, 3, and 9.
Even factors of 36 are 2, 4, 6, 12, 18, and 26.
Prime factors of 36 are 2 and 3: 36 = 2^2*3^2.

Hi, thanks for the response.
Just to clear things out,
The term “Distinct Factors” means all the Integers that are part of the Set of factors, whether prime or not.
Right?

I think I was unnecessarily confusing myself between distinct factors and distinct prime factors.
Thanks :)
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