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Re: Math: Number Theory [#permalink]
05 Mar 2010, 13:06

Bunuel wrote:

The topic is done. At last!

I'll break it into several smaller ones in a day or two.

Any comments, advises and/or corrections are highly appreciated.

What Topic are we talking abt?? _________________

Cheers! JT........... If u like my post..... payback in Kudos!!

|Do not post questions with OA|Please underline your SC questions while posting|Try posting the explanation along with your answer choice| |For CR refer Powerscore CR Bible|For SC refer Manhattan SC Guide|

Re: Math: Number Theory [#permalink]
17 Mar 2010, 13:43

Hi Bunnel,

I m confused about the extent of level for number properties.. do we have to remmeber eculer's, fermat's,wilson's theorem on prime number. Actually I found their application to be quite useful but m not sure whther there are other ways to solve the questions as well. eg difficult remainder questions and questions on HCF like if HCF of 2 numbers is 13 and their sum is 2080, how many such pairs are possible? do we see such questions on gmat? _________________

Re: Math: Number Theory [#permalink]
17 Mar 2010, 14:07

Expert's post

gurpreetsingh wrote:

Hi Bunnel,

I m confused about the extent of level for number properties.. do we have to remmeber eculer's, fermat's,wilson's theorem on prime number. Actually I found their application to be quite useful but m not sure whther there are other ways to solve the questions as well. eg difficult remainder questions and questions on HCF like if HCF of 2 numbers is 13 and their sum is 2080, how many such pairs are possible? do we see such questions on gmat?

I don't think that these theorems are needed for GMAT. _________________

Re: Math: Number Theory [#permalink]
17 Mar 2010, 14:29

So is there any way we can solve the above HCF question? Also does the number theory stated here is sufficient to cover the concepts asked? _________________

Re: Math: Number Theory [#permalink]
30 Apr 2010, 13:30

Expert's post

AloneAndInsufficient wrote:

Bunuel wrote:

NUMBER THEORY • For GMAT it's good to memorize following values: \sqrt{2}\approx{1.41} \sqrt{3}\approx{1.73} \sqrt{5}\approx{2.24} \sqrt{7}\approx{2.45} \sqrt{8}\approx{2.65} \sqrt{10}\approx{2.83}

Anyone else notice that these are wrong? They should be: • For GMAT it's good to memorize following values: \sqrt{2}\approx{1.41} \sqrt{3}\approx{1.73} \sqrt{5}\approx{2.24} \sqrt{6}\approx{2.45} \sqrt{7}\approx{2.65} \sqrt{8}\approx{2.83} \sqrt{10}\approx{3.16}

Thanks. Edited. +1 for spotting this. _________________

Re: Math: Number Theory [#permalink]
10 May 2010, 15:51

Expert's post

sag wrote:

Example: A company received $2 million in royalties on the first $10 million in sales and then $8 million in royalties on the next $100 million in sales. By what percent did the ratio of royalties to sales decrease from the first $10 million in sales to the next $100 million in sales?

Solution: Percent decrease can be calculated by the formula above: Percent=\frac{Change}{Original}*100=\frac{\frac{2}{10}-\frac{10}{100}}{\frac{2}{10}}*100=50%, so the royalties decreased by 50%.

I could not get this , i think there is some error... Plzz explain..

as the same Q in Percent Part of Math book is giving an answer of 60 %..

There was a typo. I edited it in Percent section and forgot to edit it here. Now it's OK. Thanks. +1 for spotting this. _________________

Re: Math: Number Theory [#permalink]
12 May 2010, 12:53

sag wrote:

Example: A company received $2 million in royalties on the first $10 million in sales and then $8 million in royalties on the next $100 million in sales. By what percent did the ratio of royalties to sales decrease from the first $10 million in sales to the next $100 million in sales?

Solution: Percent decrease can be calculated by the formula above: Percent=\frac{Change}{Original}*100=\frac{\frac{2}{10}-\frac{10}{100}}{\frac{2}{10}}*100=50%, so the royalties decreased by 50%.

I could not get this , i think there is some error... Plzz explain..

as the same Q in Percent Part of Math book is giving an answer of 60 %..

2 million royalties on 10 million in sales is equivalent to 20 million royalties on 100 million sales (multiply both number by 10). Going down from 20 million royalties to 8 million royalties is a decrease of 60%.

Re: Math: Number Theory [#permalink]
10 Jun 2010, 14:00

Expert's post

bely202 wrote:

If a is a factor of bc, and gcd(a,b)=1, then a is a factor of c.

Can anyone please explain this rule??? I'm not sure what it means by gcd(a,b)=1.

Thanks a bunch and great summary !!!!!

gcd(a,b)=1 means that greatest common divisor of a and b is 1, or in other words they are co-prime, the don't share any common factor but 1. So if we are told that a is a factor of bc and a and b don't share any common factors, then it must be true that a is a factor of only c.

So if a=3, b=5 (a and b don't share any common factors but 1, gcd(a,b)=1), c=6bc=30 --> a=3 is a factor of c=6. _________________