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GMAT Prep - LCM

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by ildude02 » Wed Jun 18, 2008 8:14 pm
I couldn't think of a concrete and simpler way, except for solving by using numbers. If we can find the value of x, we should be able to solve the problem.

statement 1 gives LCM of 30 ,so when cnosiderign the factors of 30, (1,2,3,5,6,10,15, 30). All the multiples of 5 can give LCM of 30. And when we substitue these multiples of 5 in the question, we always get LCM 90.

Applying the same logic for statement 2, we can find the LCM. So D the answer.

I would appreciate if there is a definite properties of LCM we could use instead of using numbers since it did take some to figure out the answer.
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by Ian Stewart » Wed Jun 18, 2008 8:49 pm
There's one thing you can learn about LCMs that makes this question quite easy:

to find the LCM of x, y, and z, you can first find the LCM of x and y, then find the LCM of this number with z.

That might sound confusing at first- the point is this: what is the LCM of 6, 8 and 10? You don't need to do it all at once. You can find the LCM of 6 and 8 first, which is 24, then find the LCM of 24 and 10 (which is 120). Or we could have found the LCM of 6 and 10 first (which is 30), then the LCM of 30 and 8 (again, we get 120). And so on.

Okay, on to the question:

The question asks, what is the LCM of x, 6 and 9.

From 1), the LCM of x and 6 is 30. By the discussion above, the question is just 'what is the LCM of 30 and 9?' Sufficient.

From 2), the LCM of x and 9 is 45. So the question is asking, 'what is the LCM of 45 and 6?'. Sufficient.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by alexdallas » Sat Jul 18, 2009 9:16 am
tks Ian
great explanation
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