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Simple MATH question but having brain fart!

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by uhsa » Wed Aug 04, 2010 6:57 am
Simple question:

* If x is an integer divisible by 15 but not divisible by 20, then x CANNOT be divisible by which of the following?
-------------6
-------------10
-------------12
-------------30
-------------150

The way I approached it is -- To be divisible by 15, but not by 20 would mean it needs to be divisible by 3, 5 but not 2, 4?

I know the answer but I am hoping someone explains to me in a simple way why/how?

Thanks,

uhsa
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by kvcpk » Wed Aug 04, 2010 7:06 am
x is divisible by 15 but not 20.
x is divisible by 5 and 3
x is not divisible by 5 and 4
Hence x should be divisible by 3 but not by 4
Hence x cannot be divisible by 3*4 = 12

Hope this helps!!
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by uhsa » Wed Aug 04, 2010 7:11 am
kvcpk,
That would mean that out of the answers given, 10, 30 and 150 are not divisible by 12?
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by kvcpk » Wed Aug 04, 2010 7:18 am
uhsa wrote:kvcpk,
That would mean that out of the answers given, 10, 30 and 150 are not divisible by 12?
I actually took an example of 30. 30 is divisible by 15 but not by 20.

Now 30 is div by 6,10,30 - ABD out
If I take x = 150, I see that x is also divisible by 150.

Hence picked C
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by kvcpk » Wed Aug 04, 2010 7:21 am
Simplest approach would be to choose X to be 150.
ABDE all divide 150.
C doesnt.
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by uhsa » Wed Aug 04, 2010 7:37 am
Thanks.
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by Gurpinder » Wed Aug 04, 2010 9:40 am
just take 150 because its divisble by 15 and not by 20.

you can also take a small number like 30, but you will have a problem with it when you go to 150. so pick the biggestest one from the options.

so 150/6 = 25
150/10 = 15
150/12 = a fraction
150/30 = 5
150/150=1.


so theres your answer.
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