Last Problem:
In the middle of a round pool lies a beautiful water-lily. The water-lily doubles in size every day. After exactly 20 days the complete pool will be covered by the lily. After how many days will half of the pool be covered by the water lily?
Answer:
19 days. Since the water-lily doubles its size every day and the complete pool is covered after 20 days, half of the pool will be covered one day before that, so 19 days.
Today’s Problem:
A snail is at the bottom of a 20 meters deep pit. Every day the snail climbs 5 meters upwards, but at night it slides 4 meters back downwards. How many days does it take before the snail reaches the top of the pit?