Games

Start by drawing a number line from 0 to 20 like this:

You can find some of these number lines here.
Player 1 chooses a number on the line and crosses it out.
Player 1 then chooses a second number and crosses that out too.
Finally, Player 1 circles the sum or difference of the two numbers and writes down the calculation.

For example, the first player’s turn could look like this:

Player 2 must start by crossing off the number that Player 1has just circled. Player 2 then chooses another number to cross out and then circles a third number which is the sum or difference of the two crossed-off numbers. Player 2 also writes down their calculation.

For example, once Player 2 has had a turn, the game could look like this:

Play continues in this way with each player starting with the number that has just been circled.

For example, Player 1 could then have a turn which left the game looking like this:

The winner of the game is the player who stops their opponent from being able to go.

Variation:

Include integer addition and subtraction by drawing the number line from -10 to +10.

Source: nrich.maths.org/6589