Learn the most important rule for subtraction: how to know when you need to 'borrow' or 'regroup.' This is the key decision you make before you start subtracting!
When you look at a subtraction problem, which column do you look at first to decide if you need to regroup?
Always start with the ones column on the far right. $$\begin{array}{rr}
& 34 \\
- & 17
\end{array}$$
When looking at the ones column, when do you need to regroup?
You need to regroup when the digit on the bottom is bigger than the digit on the top.$$\begin{array}{rr}
& 34 \\
- & 17
\end{array}$$
What's the classroom rhyme to remember when to regroup?
"More on the floor? Go next door and get ten more!"
Consider the problem \(52 - 18\). Do you need to regroup? Why?
Yes, because in the ones place, \(8\) (on the floor) is bigger than \(2\) (on the top).$$\begin{array}{rr}
& 52 \\
- & 18
\end{array}$$
Consider the problem \(76 - 33\). Do you need to regroup? Why?
No, because in the ones place, \(3\) is not bigger than \(6\). You can subtract directly.$$\begin{array}{rr}
& 76 \\
- & 33
\end{array}$$
What does the rhyme "More on top? No need to stop!" mean?
If the top digit in a column is bigger than or equal to the bottom digit, you can subtract without regrouping.
Problem: \(80 - 45\). Do you need to regroup?
Yes. In the ones place, the top digit is \(0\) and the bottom digit is \(5\). Since \(5\) is more than \(0\), you must regroup.$$\begin{array}{rr}
& 80 \\
- & 45
\end{array}$$
Problem: \(98 - 29\). Which digit tells you that you need to regroup?
The \(9\) in the ones place of \(29\), because it is greater than the \(8\) above it.$$\begin{array}{rr}
& 98 \\
- & 29
\end{array}$$
Problem: \(65 - 61\). Do you need to regroup?
No. In the ones place, \(1\) is not greater than \(5\), so you can subtract.$$\begin{array}{rr}
& 65 \\
- & 61
\end{array}$$
