Lily is a lilypad in a small pond. Lily doubles in size each day. On the 20th day she covers the whole pond. On what day was Lily half the size of the pond?

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.