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AA.1 Add three-digit numbers

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What is three-digit addition?

Three-digit addition is adding numbers that are between 100 and 999. We add the hundreds, tens, and ones places to find the total.

Example:
  • 345 + 126 = ?
Note

Always line up numbers by place value: hundreds under hundreds, tens under tens, and ones under ones.

How to add using the break apart method

The break apart method means separating numbers into hundreds, tens, and ones before adding. This makes big numbers easier to solve step by step.

Steps:
  • Take 345 + 126
  • Break apart: 345 = 300 + 40 + 5, 126 = 100 + 20 + 6
  • Add hundreds: 300 + 100 = 400
  • Add tens: 40 + 20 = 60
  • Add ones: 5 + 6 = 11
  • Put them together: 400 + 60 + 11 = 471
Note

When the ones add up to a number greater than 9, regroup into tens and ones. Example: 11 = 10 + 1.

Why use the break apart method?

Breaking apart numbers helps you see place value clearly. It shows how hundreds, tens, and ones work together in addition.

Example:
  • 218 + 137
  • Break apart: 200 + 10 + 8 and 100 + 30 + 7
  • Add hundreds: 200 + 100 = 300
  • Add tens: 10 + 30 = 40
  • Add ones: 8 + 7 = 15
  • Put together: 300 + 40 + 15 = 355
Note

This method helps you solve problems mentally and check your work more easily.

Applying three-digit addition in real life

We use three-digit addition to solve real-world problems, like adding prices, distances, or scores.

Example:
  • A school library has 235 storybooks and 148 science books. How many books are there in all?
  • Break apart: 200 + 30 + 5 and 100 + 40 + 8
  • Add hundreds: 200 + 100 = 300
  • Add tens: 30 + 40 = 70
  • Add ones: 5 + 8 = 13
  • Put together: 300 + 70 + 13 = 383 books
Note

Always check if your answer makes sense in the real-world situation. Estimating first can help.