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 lily will cover the complete pool.
After how many days will the water lily cover half of the pool?
Because 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, after 19 days.
Conclusion: after 19 days, the water lily will cover half of the pool.