Answer: C

Statement (1) is insufficient. Simplify the inequality:
640y/640 < 320/640
y < 0.5
y could be between 0.3 and 0.5, but it could also be less than 0.3.

Statment (2) is also insufficient. Again, simplify:
800y/800 > 320/800
y > 0.4
y could be between 0.4 and 0.6, but it could also be greater than 0.6.

Taken together, the statements are sufficient. If y is less than 0.5 and greater than 0.4, it must be between 0.3 and 0.6. Choice (C) is correct.