Answer:
19th day
Step-by-step explanation:
So say Lily was 1 when Lily was planted
First day Lily is 2
Second day Lily is 4
Third day Lily is 8
etc...
See the pattern?
The pattern is 2^x
On the 20th day, Lily was 2^20
So when Lily was half, Lily was 2^20 / 2
2^x = 2^20 / 2
2^x = 2^19
On the 19th day, Lily was half.
