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Question from a lab

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mph.harding
 Post subject: Question from a lab
 Post Posted: Mon Apr 05, 2010 7:18 pm 
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Course Students


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In lab 5, slide 21 a problem is presented:

If R and S are integers,is R + S divisible by 3?
(1) R is divisible by three
(2) S is divisible by three

The instructor said the answer was d (Both answers together are sufficient).

What if R is -3?
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thoppae.saravanan
 Post subject: Re: Question from a lab
 Post Posted: Tue Apr 06, 2010 10:32 am 
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If R is -3 anyway you need to know the value of S. I suppose your question is what if R is -3 and S is 3. In this case the value R+S will be 0 which is divisible by 3.

So both statements together are sufficient to answer this question.
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dinesh19aug
 Post subject: Re: Question from a lab
 Post Posted: Fri Apr 09, 2010 2:38 pm 
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Course Students


Posts: 37
This questions answer would be C - You need both statement to answer this. If you go through the first Math book i.e Number properties. It has a statement about divisibility that any number which is a multiple of X will be divisible by X even when another multiple of X is added or subtracted.

Ex- 3(call it R) is a multiple of 3 (call it X)
18(call it S) is also a multiple 3.

So 3 + 18 = 21 is also divisible by 3 or is a multiple of 3
Same holds true for -3 + 18 = 15 which is again divisible by 3.

I hope this resolves your doubt when R or S is -3.
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