Mental maths · lesson 20 of 28 · free

Add and subtract 3-digit numbers left to right — Mental Maths Mastery

Written sums start from the ones, but your head works better from the left. Keep the first number whole, and add or take away the second number one place at a time: hundreds, then tens, then ones.

How to do it in your head

Hundreds first — “add or take away the hundreds, then the tens, then the ones”

  1. Keep the first number as it is and split the second into hundreds, tens and ones.
  2. Add (or take away) the hundreds first, then the tens.
  3. Finish with the ones — bridge through a ten if you need to.

Use it for: Adding bills and marks, money in hundreds of rupees, and checking written sums.

Worked example: 647 + 285 = 932

  1. Split 285 into 200, 80 and 5.
  2. 647 + 200 = 847, then 847 + 80 = 927.
  3. 927 + 5 = 932.

Check: 932 − 285 = 647 ✓

✏️ Practice

Work it out in your head or on paper, type the answer and press Check. Stuck? Press “Show me how”.

Right: 0 · Tried: 0

🏁 Lesson 20 test

20 questions. Check each answer, then go to the next one. Get 16 or more right to pass.

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🖨️ 3-Digit Mental Addition and Subtraction Worksheet

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