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DS Question

by rakeshd347 » Thu Oct 17, 2013 8:11 pm
Is the units digit of integer x2−y2 a zero?

(1) x−y is an integer divisible by 30

(2) x+y is an integer divisible by 70
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Source: — Data Sufficiency |
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by theCodeToGMAT » Thu Oct 17, 2013 8:52 pm
(x+y)(x-y) --> INT-->doesnt means x & y are INT

Statement 1:
(x-y)/30
Suppose x-y = 60 --> x = 61.2 & y = 1.2
So, (x+y)(x-y) = 62.4 * 60 ==> _ _ 4 NO
Suppose x-y = 60 --> x = 61 & y = 1
so, (x+y)(x-y) = 62*60 ==> _ _ 0 YES
INSUFFICIENT

Statement 2:
(x+y)/70
Suppose x + y = 70 --> x = 71.2 & y = 1.2
So, (x+y)(x-y) = 72.4 * 70 ==> _ _ 4 NO
Suppose x + y = 70 --> x = 71 & y = 1
so, (x+y)(x-y) = 72*60 ==> _ _ 0 YES
INSUFFICIENT

Combining...
both (x-Y) & (x+y) has "0" as unit digit
SUFFICIENT

Answer [spoiler]{C}[/spoiler]
R A H U L
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by TheGmatTutor » Sat Oct 19, 2013 8:12 am
The way to question is written, in which x and y themselves could be decimals, the answer would be C. However, if x and y themselves have to be integers, then the answer would be A.
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