a). No. Variation in 'y' has no effect on 'x'.
b). No. Variation in 'x' has exactly the opposite effect on 'y'.
c). Yes. Variation in 'x' causes 'y' to vary 6 times as much in the same direction.
d). No. Doubling 'x' does not double 'y'.
It would take 6 days to consume a whole apple, so you multiply 6 by 3 to get your answer, 18 days :D