Math Core

You already know how to add two-digit numbers. Adding bigger numbers, like 358 + 267, works the same way. You just have one more place to take care of: the hundreds.

Add place by place

A three-digit number has hundreds, tens and ones. The number 243 is 2 hundreds, 4 tens and 3 ones, or $200 + 40 + 3$ .

To add, you can break both numbers apart and add each place on its own.

Worked example: Breaking numbers apart

Find $243 + 135$ .

Break each number into hundreds, tens and ones:

	Hundreds	Tens	Ones
243	200	40	3
135	100	30	5
Sum	300	70	8

Put the parts back together: $300 + 70 + 8 = 378$ .

When a place gets 10 or more

Sometimes a place adds up to 10 or more. Then you regroup: 10 ones make 1 ten, and 10 tens make 1 hundred.

Worked example: Adding with regrouping

Find $358 + 267$ .

Break apart: hundreds $300 + 200 = 500$ , tens $50 + 60 = 110$ , ones $8 + 7 = 15$ . Then $500 + 110 + 15 = 625$ .

Stack the numbers and add from right to left:

\begin{array}{r} \scriptstyle 1\,1\phantom{0} \\ 358 \\ +\,267 \\ \hline 625 \end{array}

Ones: $8 + 7 = 15$ . Write 5 and carry 1 ten.
Tens: $1 + 5 + 6 = 12$ tens. Write 2 and carry 1 hundred.
Hundreds: $1 + 3 + 2 = 6$ .

Both ways give 625.

Adding three-digit numbers

Line up the ones, tens and hundreds.
Add the ones, then the tens, then the hundreds.
If a place adds up to 10 or more, keep the ones digit and carry 1 to the next place to the left.

Common mistake

Don't forget the carried 1! In $358 + 267$ , the tens are $1 + 5 + 6 = 12$ , not $5 + 6 = 11$ . Write the little 1 above the next column so you remember to add it.

A mental math trick

Numbers close to a hundred are easy to add if you make them friendly first.

Worked example: Make a friendly number

Find $498 + 236$ .

498 is just 2 less than 500. Add 500 instead, then take away the extra 2:

500 + 236 = 736, \qquad 736 - 2 = 734.

So $498 + 236 = 734$ .

Check with rounding

Rounding helps you tell whether an answer makes sense. For $358 + 267$ , round each number to the nearest hundred: $400 + 300 = 700$ . Our answer, 625, is close to 700, so it is reasonable. If you had gotten 5,125 or 95, you would know something went wrong.

Tip

Before you add, make a quick estimate. After you add, compare. If the answer is far from the estimate, check your work.

Practice

Practice 1

Find $234 + 152$ .

Type a number.

Practice 2

Find $345 + 127$ .

Type a number.

Practice 3

Find $468 + 251$ .

Type a number.

Practice 4

Find $376 + 158$ .

Type a number.

Practice 5

Find $299 + 436$ .

Type a number.

Practice 6

A bakery sold 187 muffins in the morning and 245 muffins in the afternoon. How many muffins did it sell in all?

Type a number.

Practice 7

Round each number to the nearest hundred. Which is the best estimate for $412 + 379$ ?

Practice 8

Find $125 + 230 + 318$ .

Type a number.