Question

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?​

Answer 1

19th day

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.