Three-Digit Addition Examples

Mental Math · Axiom Academy

EXAMPLE Adding 347 + 285 Left to Right Break three-digit numbers into hundreds, tens, and ones, add each place value, then combine — the same left-to-right method extended one more place. Calculate 347 + 285 mentally using left-to-right addition — the same technique as the last example, now extended one more place value (hundreds). Well done — you've extended left-to-right addition to three-digit numbers, and picked up a second technique (compensation) along the way. Remember: Work left to right: start with the largest place value (hundreds) first, just like reading. Break it down: separate both numbers by place value — hundreds, tens, and ones — then add each part. Use compensation: round to a friendly number and adjust afterward; handy when a number sits close to a multiple of ten or a hundred. Keep a running total: build up the answer step by step ( 500 , then 620 , then 632 ) instead of computing everything at once. The answer: 347 + 285 = 632 — and by compensation, 463 + 279 = 742 (whether you round 279 to 280 or to 300 ). With practice, you'll start to see which strategy fits a problem best. Left-to-right addition works for every problem, while compensation shines when a number sits close to a friendly round number — often one ending in 5 , 8 , or 9 .

This is the written version of the interactive lesson above. See the full Mental Math course.