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Checking calculations using the inverse operation
The inverse calculation is used to check how many lumps of sugar have been added to a cup of tea. Two cartoon characters are drinking tea. One puts 10 lumps of sugar from a bowl into his cup. He checks how many he has added by counting how many are left in the bowl. He then adds a further 21 lumps to his cup, again checking the number he has put in his tea by counting the lumps left in the bowl.
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