Mental math strategy: use an easier problem

Instead of subtracting 29 from 66, we can subtract 30. It makes for a much easier problem to solve! Lastly, we adjust the answer, because in subtracting 30 instead of 29, we subtracted one too much.
This is a really neat strategy and works for many kinds of problems, such as 99 + 87 or 4031 - 999.
Practice two-digit mental math with these free online exercises:
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Play Hidden Picture Game and practice mental subtraction:
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This lesson is part of the Math Mammoth Grade 3 Curriculum:
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