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bww
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 Posted: Sun May 13, 2007 2:28 pm Post subject: DS prep |
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14. If x and y are integers, is xy + 1 divisible by 3?
(1) When x is divided by 3, the remainder is 1.
(2) When y is divided by 9, the remainder is 8.
Thx |
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jayhawk2001
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| Posted: Sun May 13, 2007 2:36 pm Post subject: Re: DS prep |
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| bww wrote: | 14. If x and y are integers, is xy + 1 divisible by 3?
(1) When x is divided by 3, the remainder is 1.
(2) When y is divided by 9, the remainder is 8.
Thx |
Is it C ?
1 - insufficient. x = 3t+1.
[ (3t+1)*y + 1 ] / 3
= (3ty + y + 1) / 3
So, divisibility depends on the value of y
2 - insufficient. y = 9s + 8
(xy + 1) / 3
= (9sx + 8x + 1) / 3
Again, depends on value of x
Together,
(3t+1)*(9s+8 ) + 1
= 27ts + 9s + 24t + 8 + 1
= 27ts + 9s + 24t + 9
All the terms above are divisible by 3 and hence is sufficient. |
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bww
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| Posted: Sun May 13, 2007 3:43 pm Post subject: |
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yes, OA is C
thx |
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