What strategy do I use for this problem?

Question

1. A lily pad grows so that each day it doubles its size. On the twentieth day of its life, it completely covers a pond. On what day was the pond half covered? 

Strategy used? _______________________________________

Answer 1

Use basic logic.

From one day to the next the size doubles.

So going backwards from one day to the day before it does the opposite ie halves.

So if it is covered on 20th day, it must be half covered on the day before (19th)