Back when kids had to subtract with the antiquated concept called borrowing, subtraction might have been a bit tedious. All that crossing out and rewriting could be confusing and seem disorganized on paper to some kids. When you help your kids master double digit subtraction today, even when borrowing -- or regrouping, as it is now known -- is involved, making it fun is the key to helping them understand and remember the concept.
Place value is integral to understanding double digit subtraction. When you set up a double digit subtraction problem for your child, draw vertical lines next to the ones and tens places. Use different colors for each stripe to help your child visualize the process. For example, you might use red stripes around the ones places and blue for the tens place. If you color-code stripes, your child should use the same colors for each place value every time she does this type of problem.
Your child can use blocks that snap together to make a tens unit, or other objects that can be broken down into 10 pieces to see how regrouping works. Teachers posting ideas on how to teach regrouping to students on the ProTeacher website note that using a dime and 10 pennies to demonstrate the concept is also effective. One teacher suggests using a red licorice piece and 10 marshmallows to teach regrouping. For example, give your child two dimes, and say that you want to buy an ice cream from him. The ice cream cone costs $0.17, so you can demonstrate getting 10 pennies in exchange for each dime from the “bank,” which could be another child or just a box or jar holding 20 pennies. The main idea is that when you take a tens unit from the tens place value, it is the same as 10 ones and can be used to add to the number in the ones place value. Your child should also remember that there is one fewer tens unit now in the tens stripe since the ones place just “borrowed” one from his neighbor.
An easy poem to help your child remember when to regroup from the ProTeacher website is:
More on top? No need to stop! More on the floor? Go next door. Get one ten. That's ten ones more. Numbers the same? Zero's the game!
This rhyme can help your child remember when to regroup, although it still uses the concept of "borrowing" from a neighbor. Some double digit problems do not require regrouping, so saying this chant is a good reminder of when it is necessary. Remind your child that if the smaller number in the ones stripe is on the bottom, he doesn’t have to regroup. If the smaller number is on the top, he will need to regroup.
You can also make double digit subtraction into a story. Tell your child that the smaller number on the top in the one’s place stripe needs to borrow some numbers from the tens place value next door before she can subtract. There just aren’t enough numbers to do it yet. The ones number goes next door, and borrows a tens unit from the neighbor. This is the same as 10 ones. He can then change it into ones when he gets home and add it to what he has in the ones stripe. Then your child can subtract.