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Reasoning based on this Japanese activity.

Strike it Out for Two

Stage: 1 and 2 Challenge Level: Challenge Level:1

Here's a game to play with a grown-up!

How do you play?
You'll need a grown-up to play with.
You'll also need a number line from 0 to 20, like the one above. You can find some of these here.

The grown-up chooses a number on the line and crosses it out.
They then choose a second number and cross that out too.
Finally, the grown-up circles the sum or difference of the two numbers and writes down the calculation.

For example, the grown-up's go could look like this:

You must start by crossing off the number that the grown-up has just circled.
You then choose another number to cross out, and then circle a third number which is the sum or difference of the two crossed-off numbers.
You also writes down their calculation.

For example, once you have had a turn, the game could look like this:


For example, the grown-up could then have a turn which left the game looking like this:


What is your strategy for winning?
What is the biggest number of numbers you can cross out?

Notes for grown-ups
Strike it Out offers an engaging context in which to practise addition and subtraction, but it also requires some strategic thinking. It is easily adaptable and can be used co-operatively or competitively.

Easier version: try starting with a number line from 0 to 10 instead.
Harder version: try using multiplication and division as well as addition and subtraction. Children could suggest different number lines that they could use: maybe longer number lines, or ones involving decimal or negative numbers.

There's a classroom version of this game here.