Strike it Out

It's a good idea to play the game lots of times to start with to get the feel of it.
You could keep the lines you use for each game and compare them afterwards.

Are there any numbers that you haven't crossed off in any of the games you've played on the 0-20 number line? Will it ever be possible to cross them off? 

When you come to consider the 1-20 number line, have a think about the total number of numbers you use in a game.