When we teach children addition on paper, we teach them to start on the right (the ones column) and carry over to the left. While that is great for written pencil-and-paper math, it is terrible for mental math. When you calculate in your head, working from left to right is much faster and easier to hold in your short-term memory.
Why Left to Right Works Better
When you read a number like 452, you read the hundreds first, then tens, then ones. Mental math works best when it follows the natural order of reading numbers.
Step-by-Step Example: 64 + 28
Instead of thinking 4 + 8 = 12, carry the 1..., do this:
- Break 28 into 20 + 8.
- Add the tens first:
64 + 20 = 84. - Now add the ones:
84 + 8 = 92.
You only had to hold one number in your head at a time (84, then 92). No carrying marks needed!
Three-Digit Example: 345 + 236
Break the second number into hundreds, tens, and ones:
- Start with 345.
- Add hundreds:
345 + 200 = 545 - Add tens:
545 + 30 = 575 - Add ones:
575 + 6 = 581
Final answer: 581. With just a week of practice, middle school students can do three-digit additions in under 5 seconds using this method.