Subtraction with regrouping is necessary when the digit in the (top number) is smaller than the digit in the subtrahend (bottom number) within the same place value column.
Using visual "rods" (tens) and "units" (ones) to physically show the "break down" of a ten. Subtraction with Regrouping - Math Video for 2nd Grade
Videos often use base-ten blocks or place value charts to show that the value of the number remains the same (e.g., changing 42 from "4 tens, 2 ones" to "3 tens, 12 ones"). 2. Key Instructional Strategies Subtraction with regrouping is necessary when the digit