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:
https://www.mathmammoth.com/practice/...
Play Hidden Picture Game and practice mental subtraction:
https://www.mathmammoth.com/practice/...
This lesson is part of the Math Mammoth Grade 3 Curriculum:
https://www.mathmammoth.com/complete/...