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Prime numbers

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by aippingw » Mon Apr 12, 2010 5:05 am
Hi guys !

I think I would need your help with prime numbers.

I'm studying with MGMAT. I think I got something wrong about Primes because I am a little bit confused. Here's my problem

135
Primes:3,3,3,5

225
Primes:3,3,5,5

135*125=30375
Primes:3,3,3,3,3,5,5,5 (Let me know if I am wrong but I don't think so)

BUT on MGMAT study guide, on Problem was
J is divisible by 10 and 12 so what is J divisible by:

10
Primes:5,2

12
Primes:3,2,2

So J
Primes 5,2,2,3 + other numbers

Here is the thing I don't understand. MGAMT says that it shouldn't be 5,2,2,2,3 + other numberes because there is one 2 that is redundant.

I got this but then, what is the rule? When do you know that primes can be redundant or not ?
I was thinking maybe because un the first case, where the primes is not redundant, it is because it is a multiplication but when it redundant it is because the problem says J is divisible by 10 "AND" 12.

A little help will be really appreciated! Thanks

Andrew
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Source: — Quantitative Reasoning |
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by srinivasarajui » Mon Apr 12, 2010 9:16 am
When it is said the a number is divisible by 10 and 12, It need not be 10*12*some thing else. It can just be the LCM. So they just calculated LCM.
For Example 60 is divisible by both 10 and 12, here 60 is also the LCM So they found the prime factors of the LCM.
Srinu
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by aippingw » Mon Apr 12, 2010 10:06 am
So basically when a question contains pretty much "AND" I should be looking for the Primes in the Venn Graph (cf MGMAT Guide #1) I use for the GCF. Correct ?

Thx for the reply !
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