1st day it jumps 5 meters and falls 4 meters, now will be at a height of 1 meter.
2nd day it jumps to 6 meters and falls back to 2 meter.
3rd day it jumps to 7 meters and falls back to 3 meter.
15th day it jumps to 19 meters and falls back to 15 meters.
Finally, 16th day it jumps to 20 meters, but now the frog have reached the top of the well.
Frog reaches top of the well at 16th day.