Two easy sums beat one hard one
Adding 58 + 34 all at once feels hard. But 50 + 30 and 8 + 4 are both trivial. Decomposition turns one scary problem into two friendly ones.
Why it works
You already know 5 + 3 and 8 + 4. Place value just adds zeros.
Mental Math · Addition · A1
Add the big parts first.
To add two numbers in your head, split each into place values, add the tens to the tens and the ones to the ones, then put the parts together. Starting from the big parts gives you an instant estimate and keeps everything organized.
How to think about it.
Watch the method
Add the big parts first
58 + 34 isn't one hard sum — it's two easy ones. Add the tens (50 + 30 = 80), add the ones (8 + 4 = 12), then combine (80 + 12 = 92).
= ?
1
Add the tens.
2
Add the ones.
3
Add those parts together for the answer.
Practice until it feels easy.
Your turn
Solve it in your head.
Use the method, then check. Stuck? Reveal the steps.
streak 0
best 0
… =
The idea underneath.
From easy to tricky.
46 + 23
1
Tens
40 + 20 = 60.
2
Ones
6 + 3 = 9.
3
Combine
60 + 9 = 69.
58 + 34
1
Tens
50 + 30 = 80.
2
Ones
8 + 4 = 12.
3
Combine
80 + 12 = 92.
348 + 275
1
Hundreds
300 + 200 = 500.
2
Tens
40 + 70 = 110.
3
Ones
8 + 5 = 13.
4
Combine
500 + 110 + 13 = 623.
Where students slip.
Trap
Add the tens and forget the ones can make another ten.
Fix
8 + 4 = 12 — that is a ten and two ones. Combine carefully: 80 + 12 = 92, not 82.
Trap
Mixing up places, e.g. adding the 5 to the 4.
Fix
Tens go with tens, ones go with ones. Keep the columns straight in your head.
Test yourself.
The Test
12 problems across three tiers. Auto-graded on submit. Hints and a full walkthrough on every question. Your work is saved on this device — sign in to keep it across devices.
0/12Answered
How to answerType just the value. Formatting won't trip you up — we ignore spaces, capital letters, commas, and subscript style, so
t₁ = 5, d = 3 and t1=5,d=3 both count. For a list of numbers, separate them with commas (e.g. 3, 10, 17).Core
Core (4)
C1
58 + 34 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
C2
46 + 23 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
C3
35 + 27 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
C4
72 + 19 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
Apply
Apply (4)
A1
145 + 38 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
A2
64 + 58 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
A3
137 + 45 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
A4
256 + 33 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
Extension
Extension (4)
E1
348 + 275 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
E2
467 + 286 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
E3
588 + 234 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.
E4
675 + 248 = ?
Need a hint?
- Split both numbers into tens and ones.
- Add the tens, add the ones, then combine the two totals.