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If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

Remember a property of LCM and GCF: If x and y are two positive integers, LCM * GCF = x* y Stmnt 1: Just the GCF will give you the product of the two numbers. Not their individual values. Stmnt 2: Knowing the value of y and LCM is not sufficient to get x. x could be 840 or 105 or 5 etc.

Using both together, you get x = 840*56/168 = 280 Answer (C). _________________

If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

Remember a property of LCM and GCF: If x and y are two positive integers, LCM * GCF = x* y Stmnt 1: Just the GCF will give you the product of the two numbers. Not their individual values. Stmnt 2: Knowing the value of y and LCM is not sufficient to get x. x could be 840 or 105 or 5 etc.

Using both together, you get x = 840*56/168 = 280 Answer (C).

Karishma is this a known property? I have never ran across it before? I am not sure I understand why it works either. Can you please explain?

If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

Remember a property of LCM and GCF: If x and y are two positive integers, LCM * GCF = x* y Stmnt 1: Just the GCF will give you the product of the two numbers. Not their individual values. Stmnt 2: Knowing the value of y and LCM is not sufficient to get x. x could be 840 or 105 or 5 etc.

Using both together, you get x = 840*56/168 = 280 Answer (C).

Karishma is this a known property? I have never ran across it before? I am not sure I understand why it works either. Can you please explain?

It is taught at school (though curriculums across the world vary)

Let us take an example to see why this works:

\(x = 60 = 2^2 * 3 * 5\) \(y = 126 = 2*3^2*7\)

Now GCF here will be \(6 (= 2*3)\) (because that is all that is common to x and y) LCM will be whatever is common taken once and the remaining i.e. \((2*3) * 2*5 * 3*7\)

When you multiply GCF with LCM, you get \((2*3) * (2*3 *2*5 * 3*7)\) i.e. whatever is common comes twice and everything else that the two numbers had.

I can re-arrange this product to write it as \((2*3 * 2*5) * (2*3 * 3*7)\) i.e. 60*126

This is the product of the two numbers. Since GCF has what is common to them and LCM has what is common to them written once and everything else, GCF and LCM will always multiply to give the two numbers.

Can you now think what will happen in case of 3 numbers? _________________

If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

The property Karishma used is often tested on GMAT. So, it's a must know property: for any positive integers \(x\) and \(y\), \(x*y=GCD(x,y)*LCM(x,y)\).

Re: If the least common multiple of integers x and y is 840, [#permalink]
27 Sep 2012, 17:41

The question does not say the integers are positive. Is it implied? Factors are always positive so are GCDs and LCMs, but x could be a positive or a negative integer?

Now GCF here will be \(6 (= 2*3)\) (because that is all that is common to x and y) LCM will be whatever is common taken once and the remaining i.e. \((2*3) * 2*5 * 3*7\)

When you multiply GCF with LCM, you get \((2*3) * (2*3 *2*5 * 3*7)\) i.e. whatever is common comes twice and everything else that the two numbers had.

I can re-arrange this product to write it as \((2*3 * 2*5) * (2*3 * 3*7)\) i.e. 60*126

This is the product of the two numbers. Since GCF has what is common to them and LCM has what is common to them written once and everything else, GCF and LCM will always multiply to give the two numbers.

Can you now think what will happen in case of 3 numbers?

You've got to be kidding me....that makes perfect sense. I'm glad I'm taking the GMAT, I'm learning all these fascinating formulas I've never used before.

If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

The property Karishma used is often tested on GMAT. So, it's a must know property: for any positive integers \(x\) and \(y\), \(x*y=GCD(x,y)*LCM(x,y)\).

Is there a sheet with the 'must-know' type stuff on it? I know there's the PDF you guys put together, but is there anything that is just a one or two page listing of the commonly used formulas like this?

Is there a sheet with the 'must-know' type stuff on it? I know there's the PDF you guys put together, but is there anything that is just a one or two page listing of the commonly used formulas like this?

The list of must-know formulas would be really short and you would know most of the formulas on it (e.g. Distance = Speed*Time, Sum of first n positive integers = n(n+1)/2, area of a circle = pi*r^2 etc). Even if there are a couple that you don't know, you will come across them while preparing so just jot them down.

There will be many more formulas that you could find useful in particular questions but you can very easily manage without them. Also, learning too many formulas creates confusion about their usage - when to use which one - and hence they should be avoided. _________________

Re: If the least common multiple of integers x and y is 840, [#permalink]
18 Jun 2014, 22:39

VeritasPrepKarishma wrote:

rxs0005 wrote:

If the least common multiple of integers x and y is 840, what is the value of x?

(1) The greatest common factor of x and y is 56. (2) y = 168

Remember a property of LCM and GCF: If x and y are two positive integers, LCM * GCF = x* y Stmnt 1: Just the GCF will give you the product of the two numbers. Not their individual values. Stmnt 2: Knowing the value of y and LCM is not sufficient to get x. x could be 840 or 105 or 5 etc.

Using both together, you get x = 840*56/168 = 280 Answer (C).

Hi Karishma,

Stmnt 2: if we know the value of y, and the least common multiple, i can't think of another number that X could be other than 5. Can you give an example?

Re: If the least common multiple of integers x and y is 840, [#permalink]
18 Jun 2014, 22:50

Expert's post

ronr34 wrote:

Hi Karishma,

Stmnt 2: if we know the value of y, and the least common multiple, i can't think of another number that X could be other than 5. Can you give an example?

I have given some values that x can take from statement 2 - "x could be 840 or 105 or 5 etc" Note that we don't know the value of GCF from stmnt 2 alone. Hence, x could take many different values. Only when we know the GCF too, can we find the value of x. _________________

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