Yeah, I was thinking I wouldn't redo many problems that were boring the first time without a serious incentive, too! I'd be tempted to suggest writing a program that took as input the variable numbers in the problem and gave as output the correct answer, where that's practical; this could be quicker if there are 40 problems, besides being metacognitively interesting.

Dandy, does your DS do maths competitions? For my DS, these raise the stakes in the right way to encourage carefulness.

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