Ok let's go with a relatively easy one:
There's a creature which doubles its size every day. If the creature is put in a tank, it'll fill it in 10 days. How long does it take for it to fill the 1/4th of it and how long does it take to fill the half of it?
the anwer is 8 days to fill 1/4 and 9 days to fill half, which will result in filling it completely in 10 days.