Double Digit Addition and Subtraction
Lessons in this chapter
About Double Digit Addition and Subtraction
Together and Apart - Double Digit is the biggest chapter of 1st grade and the one with the most to go wrong, because this is where regrouping arrives.
Add 2-Digit Numbers Without Regrouping and Subtract 2-Digit Numbers Without Regrouping come first, so kids meet the column layout while each column still works on its own. Getting the method secure before the complication is the whole reason for the split.
Add 2-Digit Numbers With Regrouping then introduces carrying. When the ones column makes ten or more, that ten moves into the tens column, which is only sensible to a child who understands from Numbers to 120 that the tens column counts groups of ten.
Subtract 2-digit numbers with Regrouping is the harder twin and the usual sticking point. Borrowing means breaking one ten into ten ones, and the common error, always taking the smaller digit from the larger regardless of which is on top, comes from following the layout without that idea.
Solve Word Problems on Addition and Subtraction of 2-Digit Numbers finish the chapter by putting the methods into context, which is where kids have to choose the operation rather than being handed it.
This is the longest chapter in 1st grade and it carries six worksheets, one per lesson, all worked in the page. Add 2-Digit Numbers With Regrouping and Subtract 2-digit numbers with Regrouping are where most of the mistakes happen, and the two word problem lessons at the end give 1st graders math problems where they have to choose the operation themselves. Free to try, with no login needed for your first 3.
Frequently asked questions
Six lessons, deliberately ordered. Adding 2-digit numbers without regrouping comes first, then with regrouping, then the same pair for subtraction, and the chapter closes with word problems on each operation. The two-digit addition sits in Number and Operations in Base Ten (1.NBT) and the word problems in Operations and Algebraic Thinking (1.OA); two-digit subtraction with regrouping is a second grade standard in the Common Core, so that part runs ahead.
Trading between columns. In addition, ten ones become one ten and move left, which is carrying. In subtraction, one ten is broken into ten ones and moves right, which is borrowing.
The most common error is subtracting the smaller digit from the larger in each column, whichever is on top, so 42 − 27 becomes 25. It comes from following the layout without understanding that borrowing is breaking a ten apart.
So the column method itself is secure before the complication arrives. A child fighting both the layout and the trade at once usually learns neither.
Completely. Carrying only makes sense if the tens column counts groups of ten, which is what Numbers to 120 establishes. That is why it comes first.
Taking one ten from the tens column and turning it into ten ones so the ones column has enough to subtract from. The tens digit drops by one and the ones digit rises by ten.