What is this skill?
Adding three-digit numbers means adding numbers like 346 + 278, with totals up to 1,000. Second graders add hundreds to hundreds, tens to tens and ones to ones. They use drawings of base-ten blocks, split numbers into parts (300 + 200, 40 + 70, 6 + 8), and the written column method.
The new challenge compared with two-digit addition is that regrouping can happen twice. In 346 + 278, the ones make 14, so one ten is carried. Then the tens make 4 + 7 + 1 = 12 tens, so one hundred is carried. The hundreds make 3 + 2 + 1 = 6, and the total is 624. Each regroup follows the same rule: ten of one place make one of the next.
Why it matters
This is where the place value ideas from second grade come together. If a child can carry from the tens into the hundreds as confidently as from the ones into the tens, they have understood that the pattern repeats at every place, and they will be ready for numbers of any size.
Three-digit addition also prepares for third grade, where children are expected to add and subtract within 1,000 fluently, and for multi-digit multiplication later, which involves a lot of adding with carrying. Estimating first, such as noticing that 346 + 278 is close to 350 + 300, gives children a way to catch big mistakes on their own.
Common mistakes
- Carrying from the ones but not from the tens, or the other way around. Checking each column with "Is it 10 or more?" catches both.
- Writing all of a column total under the line, such as writing 12 in the tens column, which turns the answer into a four-digit number. Remind your child that each column gets only one digit and the extra moves to the next place.
- Lining numbers up on the left instead of the right when one number has fewer digits. Right-aligned columns, or squared paper, keep ones under ones.
- Adding the carried digit twice, or forgetting it entirely. Writing small carried digits clearly at the top of the column helps.
How to help at home
- Use play money with dollar bills, dimes and pennies to add amounts like $3.46 and $2.78. Trade ten pennies for a dime and ten dimes for a dollar.
- Ask your child to estimate first by rounding to the nearest hundred, then compare the estimate to the exact answer.
- Add real numbers: the pages in two books, points in a game across several rounds, or the steps on a step counter for two days.
- Have your child explain each carry out loud: "Fourteen ones is one ten and four ones." Explaining is often where understanding settles in.
Worked examples
These are real questions from the Tutorsaurus adding three-digit numbers practice, with the answers worked out.
Example 1
760 + 116 = ?
Answer: 876. 760 + 116 = 876. Add ones, then tens, then hundreds, carrying when needed.
Example 2
374 + 556 = ?
Answer: 930. 374 + 556 = 930. Add ones, then tens, then hundreds, carrying when needed.
Example 3
114 + 219 = ?
Answer: 333. 114 + 219 = 333. Add ones, then tens, then hundreds, carrying when needed.