I show two simple examples of subtracting 3-digit numbers mentally. We can use our knowledge of place value to do these subtractions. For example, to subtract 652 - 30, notice that we're subtracting three tens. So, we look at the TENS digit in 652 and subtract 5 - 3 (in the tens).

If a child understands place value, subtractions such as 478 - 20 or 783 - 200 are easy. If a child has difficulty with them, it typically indicates the child has not understood the concept of place value (hundreds, tens, ones).

Practice mental math with three-digit numbers with this fun game (Mathy's Berry Picking Adventure):
https://www.mathmammoth.com/practice/...

Another game to practice mental subtraction:
https://www.mathmammoth.com/practice/...

This lesson is part of the Math Mammoth Grade 3 curriculum:
https://www.mathmammoth.com/complete/...