


ThreeDigit Addition and Subtraction With RegroupingIn making the transition from adding and subtracting twodigit numbers to threedigit numbers, children will build on what they have already learned about place value and regrouping. It is important to remind children of the need for proper alignment of numbers when adding and subtracting numbers of more than one digit.
Begin with threedigit addition using basic facts and mental math. This will give children a chance to use what they already know. For example, to solve a basic threedigit addition problem, use numbers that involve basic addition facts, such as
Using baseten blocks is a good way to help children visualize what they are doing when adding or subtracting threedigit numbers with regrouping. To solve an addition problem, regrouping ones, model the addends using baseten blocks. The example below shows Remind children that to add or subtract, they should begin with the ones. In this case, group 5 ones and 7 ones to make 12 ones, regroup the ones as 1 ten, 2 ones, add the tens to make 6 tens, and add the hundreds to make 2 hundreds. The answer to the problem is 262.
The same process takes place when regrouping 10 tens, except regrouped tens become 1 hundred. The example below shows In this case, the ones do not need regrouping. Add to get 5 ones. Then group 5 tens and 7 tens to make 12 tens, and regroup the tens as 1 hundred, 2 tens. Add the hundreds. The answer to the problem is 325.
The same principles can be used in threedigit subtraction, except that regrouping works in reverse. One ten becomes 10 ones when regrouped. One hundred becomes 10 tens. Use the baseten blocks to illustrate regrouping tens in threedigit subtraction for the problem
Model this same method to illustrate regrouping hundreds to solve Understanding these regrouping principles prepares children for learning horizontal addition and subtraction and for addition and subtraction of money. When teaching children to add and subtract horizontally, model how to rewrite the problem vertically. Make sure that children keep each digit in the correct place when rewriting numbers. Looking at the problem vertically makes it easier to see the hundreds, tens, and ones and to regroup if necessary. The example below illustrates how to rewrite a horizontal problem vertically in order to make it easier to solve. Adding and subtracting with money is the same as regular threedigit addition and subtraction. Writing the amount $1.23 actually is the same as writing 123, except for the dollar sign and the decimal point. Children can think of dollars as hundreds, dimes as tens, and pennies as ones. You can regroup 1 dollar as 10 dimes and 1 dime as 10 pennies, and viceversa.
Estimating is a good way to get a sense of the answer to an addition or subtraction exercise without actually computing. It also helps you see whether any answer you get is reasonable. To estimate a sum or difference, round numbers to the greatest place. The following example shows an estimate of  
Rounding each number to the nearest hundred, you find that the estimate of 
