You may hear your child and her teacher talk about Place Value Disks. When learning place value, addition, and subtraction, place value disks are a great tool to tie together your child’s understanding before using the standard algorithm.
They are especially helpful when regrouping, that is, changing one unit to another unit. Watch this tool in action on a place value chart. Forty-three minus sixteen. Place value disks help your child to see forty-three as four tens and three ones.
We know to subtract ones from ones first. Three ones take away six ones… But wait – We don’t have six ones to take! We need to regroup. Your child knows one ten is equal to ten ones. She will trade in one ten for ten ones. Now she can take away six ones! There are seven ones left.
Next, subtract tens from tens. Three tens take away one ten is two tens. Two tens and seven ones is another way to say twenty-seven!
Sometimes you need to regroup the hundreds and the tens. Three-hundred fifty-two minus one-hundred eight-five equals… umm… Place value disks can help us figure it out!
Subtract ones from ones first. But we can’t take away five ones! We only have 2 ones! So, let’s regroup. Trade in one ten for ten ones. Now we can take away five ones. Seven ones are left.
Next, subtract tens from tens. We have four tens and we need to take away eight tens. Look at that! We get to regroup again! Trade in one hundred for ten tens. Now we can take away eight tens. Six tens are left.
Last, subtract hundreds from hundreds. Two hundreds take away one hundred equals one hundred.
Note that we don’t “borrow” anything. We just regroup what we have in a different way so we can solve the problem. This will lead to some seriously strong mental math skills in your child.
And that is good to know.
Common Core Grade 2 Standard Number & Operations in Base Ten: Use place value understanding and properties of operations to add and subtract.
(2.NBT.7) Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method. Understand that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.
(2.NBT.9) Explain why addition and subtraction strategies work, using place value and properties of operations.