It can grow to 20 cm in 30 days.

29 days can be as long as 20/2= 10 cm.

28 days can be as long as 10/2 = 5cm.

2. Push back method:

Reach 4000 in 20 days.

19 days to 4000/2=2000.

18 days to 2000/2= 1000.

17 days 1000/2=500.