There is a creature which could double its size every day. So, if the creature is put in a tank then it will fill the tank in 10 days.
9 days to fill 1/2 and 8 days to fill 1/4 of the tank. If the creature fills the tank in 10 days and it doubles every day, on the ninth day it would fill 1/2 the tank. Thus on the 8th day it will fill 1/4 of tank.