Data Sufficiency

anastasios1984 wrote:Is Z<0?

i) x*y>0 , z*y>0
ii)x>0

Statement 1: xy > 0, x and y are of same sign
zy > 0, z and y are of same sign

Hence, x, y, and z are all of same sign.
As we know nothing about the sign of either x or y, we can't say whether z < 0 or not.

Not sufficient

Statement 2: We don't know what is the relation between x and z. So we can't say whether z < 0 or not.

Not sufficient

1 & 2 Together: Now we know, x is positive and z is of same sign as of x. Hence, z > 0.

Sufficient

The correct answer is C.