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Re: Let n~ be defined for all positive integers n as the remainder when (n [#permalink]

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16 Apr 2017, 18:09

1

This post received KUDOS

Bunuel wrote:

Nunuboy1994 wrote:

Ok so I was initially not able to do this problem because I didn't understand how to divide 31!/32...

Now that you guys have made the problem more clear I have a question-

5!/15 must be an integer? Because 5x4x3x2x1 contains the factors of 15 (5 x 3)

When I do 17!/30 in a calculator the result is not an integer?

First of all, 17!/30 = 11856247603200 = integer, but what 17!/30 has to do with this problem?

I trying to demonstrate the concept in this problem, which is new to me, in a different example- I think the calculator's notation just makes it appear as 1.something^e even though that doesn't necessarily mean it's not an integer. It's clear now.

Ok so I was initially not able to do this problem because I didn't understand how to divide 31!/32...

Now that you guys have made the problem more clear I have a question-

5!/15 must be an integer? Because 5x4x3x2x1 contains the factors of 15 (5 x 3)

When I do 17!/30 in a calculator the result is not an integer?

First of all, 17!/30 = 11856247603200 = integer, but what 17!/30 has to do with this problem?

I trying to demonstrate the concept in this problem, which is new to me, in a different example- I think the calculator's notation just makes it appear as 1.something^e even though that doesn't necessarily mean it's not an integer. It's clear now.

For big numbers do not user calculator (unless it's not advanced) or Excell, use Wolframalpha: https://www.wolframalpha.com/

Re: Let n~ be defined for all positive integers n as the remainder when (n [#permalink]

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27 Jun 2017, 15:09

shasadou wrote:

Let n~ be defined for all positive integers n as the remainder when (n - 1)! is divided by n.

What is the value of 32~ ?

A. 0 B. 1 C. 2 D. 8 E. 31

Simple- all this question is asking is if you have 31!/32 then what is the remainder? You don't necessarily have to expand 31!- 32 fits in because you have 16 and 2 so there is no remainder

Re: Let n~ be defined for all positive integers n as the remainder when (n [#permalink]

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09 Jul 2017, 03:12

shasadou wrote:

Let n~ be defined for all positive integers n as the remainder when (n - 1)! is divided by n.

What is the value of 32~ ?

A. 0 B. 1 C. 2 D. 8 E. 31

\(= \frac{(32-1)!}{32}\)

\(= \frac{31!}{32}\)

Numbers \(8 * 4 = 32\), will cancel out the denomintor and hence the reminder will be ZERO.

Hence, Answer is A _________________

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32! has various multiples of 32 built in say 8 * 4, 16* 2. Will not various no of multiples affect my remainder? For eg, even if 8*4 will cancel out 32 in denominator leaving remainder = 0, I still have one multiple of 16*2, correct?

Let n~ be defined for all positive integers n as the remainder when (n - 1)! is divided by n.

What is the value of 32~ ?

A. 0 B. 1 C. 2 D. 8 E. 31

32~ = (32 - 1)!/32 = 31!/32 = 31!/2^5

Since we can safely say that there are at least five 2s in 31! (for example, 31! has the factors 16 = 2^4 and 8=2^3), the remainder is zero.

Alternate Solution:

32~ = (32 - 1)!/32 = 31!/32 = 31!/2^5

We want to know the remainder when 31! is divided by 2^5. If we can establish that there are at least 5 factors of 2 in 31!, then we will know that 2^5 evenly divides into 31!, which means that the remainder would be 0. Let’s determine if we can find at least 5 twos in 31!:

31! = 31 x 30 x 29 x 28 x 27 x 26 x 25 x 24 x … x 1

31! = 31 x (2 x 15) x 29 x (2 X 2 x 14) x 27 x (2 x 13) x 25 x (2 x 2 x 2 x 3) x … x 1

Notice that we have found seven 2s, which is two more than what we needed. Thus, we know that 2^5 will evenly divide into 31!, leaving a remainder of 0.

Answer: A
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GMAT Quant Self-Study Course 500+ lessons 3000+ practice problems 800+ HD solutions

Re: Let n~ be defined for all positive integers n as the remainder when (n [#permalink]

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30 Nov 2017, 22:24

Hi,

So basically as long as there are enough prime factor powers in the numerator to cancel out all the prime factor powers in the denominator, the remainder is 0 correct?

So basically as long as there are enough prime factor powers in the numerator to cancel out all the prime factor powers in the denominator, the remainder is 0 correct?

Yes. If the numerator contains the same (or more) primes as denominator and the (positive integer) powers of primes in the numerator are at least as big as the powers of the same primes in the denominator, then the result of the division will be an integer, so the remainder will be 0.
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