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'Strike it Out' printed from http://nrich.maths.org/
Why play this game?
offers an engaging context for practising addition and subtraction, but it also requires some strategic thinking. It is easily adaptable, and can be used co-operatively or competitively.
You could click through this powerpoint presentation one 'go' at a time, asking children to watch carefully and then to talk to a partner about what they think the rules are. After the first 'go', take some suggestions, but don't say whether they are correct or not.
Instead, click through the second 'go' and give pupils more time to talk to each other about the rules again - their initial thoughts will perhaps need adapting. Repeat this process once more and then discuss the rules so that everyone is clear.
Give children time to play several games in pairs so they get a feel for it. You could share their strategies and then ask them whether they think it might be possible to cross off all the numbers in a game. Give them time to work co-operatively with their partner on this challenge before bringing them together again to see what they have found out. Some will have realised that it is
impossible to cross off zero - encourage them to explain why this is the case.
Learners could then investigate whether it is possible to cross off all the numbers if the number line goes from $1$ to $30$ instead. Many will be able to reason that it is still not possible due to there being an even number of numbers in total. Exploring the longest possible string of calculations is interesting and the children can be asked to examine why they run out of
Have you found any good ways to beat your opponent?
Can you cross out all the numbers in one game? How do you know?
What is the biggest number of numbers you can cross out?
Children can suggest their own 'what if ...?' questions, for example:
What if we could use multiplication/division?
What if we drew a longer number line?
What would happen if we included decimal numbers in our number line?
What if the number line extended beyond zero to negative numbers?
The possibilities are endless.
If children are struggling with the calculations, a shorter number line may be appropriate so focusing on a number line to 10 still elicits many of the same ideas about possibilities and outcomes as well as the way in which the operations of addition and subtraction work on a limited set of numbers.