Adding and subtracting in your head
Not every sum needs a paper and pencil. With a few tricks you can solve many 3-digit sums in your head.
Trick 1 — full hundreds are easy
When both numbers end in 00, just add the hundreds:
400 + 300 = 700
800 − 500 = 300
600 + 400 = 1 000
It's the same as 4 + 3 = 7, just spoken with hundreds.
Trick 2 — jump by 100
To add 100 to any number, only the hundreds digit changes:
247 + 100 = 347
583 − 100 = 483
You can chain it: 247 + 200 is two jumps of 100 → 447.
Trick 3 — round and adjust
For numbers that are close to a friendly hundred, round first, then fix.
199 + 348 — think 200 + 348 = 548, then take 1 back → 547.
502 − 198 — think 502 − 200 = 302, then add 2 back → 304.
This trick works because subtracting too much means you have to give some back.
Trick 4 — split into hundreds, tens and ones
Break each number into pieces and add the pieces.
234 + 152
= (200 + 100) + (30 + 50) + (4 + 2)
= 300 + 80 + 6
= 386
It looks like more steps, but each one is easy.
When to skip mental math
If the numbers are messy — say 487 + 256 with carrying in every column — written column addition is faster and safer. Use the trick that fits the numbers.
Try it
- ➕ Add and subtract to 1,000 — practise the tricks
- 🔍 Find the missing number — train both directions