DS-16
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(1) 3y^2 + 7y = x
If we divide both sides by y, we get:
3y + 7 = x/y
Since we know that y is an integer, we know that 3y + 7 is an integer. Therefore, x/y is an integer (and x is a multiple of y). Sufficient.
note: since we know that y>1, y can't be 0, so we can safely divide both sides by y.
(2) x^2 - x is a multiple of y
we can rewite this as:
x(x-1) is a multiple of y
For this to be true, either x or (x-1) has to be a multiple of y. So, x might be a multiple of y, but it doesn't have to be. Insufficient.
(1) is suff, (2) is insuff... choose (A)
If we divide both sides by y, we get:
3y + 7 = x/y
Since we know that y is an integer, we know that 3y + 7 is an integer. Therefore, x/y is an integer (and x is a multiple of y). Sufficient.
note: since we know that y>1, y can't be 0, so we can safely divide both sides by y.
(2) x^2 - x is a multiple of y
we can rewite this as:
x(x-1) is a multiple of y
For this to be true, either x or (x-1) has to be a multiple of y. So, x might be a multiple of y, but it doesn't have to be. Insufficient.
(1) is suff, (2) is insuff... choose (A)
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