You need: a number line marked with numbers 0-20, a pencil or pen.
The first player chooses a number on the line and crosses it out. The same player then chooses a second number and crosses that out too. Finally, he or she circles the sum or difference of the two numbers and writes down the calculation (for example cross out 4, cross out 12 and circle 16, because 4+12=16). The second player must start by crossing off the number that player 1 has just circled (in the example above they would start on 16, while in the picture example they would start on 11).
He or she 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 (for example cross off 16, cross of 2, circle 14, because 16-2=14).
Player 2 also writes down their calculation. The winner is the person who stops the other from being able to go/ or if you use all numbers!