Mental Math · Decimals · Add
Adding decimals is just adding in parts, with the decimal point as a guide. Line up the points, add the whole-number parts, add the decimal parts, and combine — exactly the left-to-right habit, now with a point.
Watch the method
3.4 + 2.8: add the wholes (3 + 2 = 5), add the decimals (0.4 + 0.8 = 1.2), then combine: 5 + 1.2 = 6.2.
Add the whole-number parts.
Add the decimal parts.
Put them together.
Your turn
Use the method, then check. Stuck? Reveal the steps.
A decimal number is whole + tenths + hundredths. Add each column to its match. Lining up the points guarantees tenths add to tenths, not to whole numbers.
Why it works
3.4 is 3 wholes and 4 tenths — add the wholes and the tenths separately, like dollars and dimes.
5.6 + 1.3
Wholes
5 + 1 = 6.
Decimals
0.6 + 0.3 = 0.9.
Combine
6 + 0.9 = 6.9.
3.4 + 2.8
Wholes
3 + 2 = 5.
Decimals
0.4 + 0.8 = 1.2.
Combine
5 + 1.2 = 6.2.
12.5 + 3.7
Wholes
12 + 3 = 15.
Decimals
0.5 + 0.7 = 1.2.
Combine
15 + 1.2 = 16.2.
Ignoring the point and adding 34 + 28.
Keep the place values: 3.4 + 2.8 = 6.2, not 62. The point can't be dropped.
Forgetting to carry when decimals exceed 1.
0.4 + 0.8 = 1.2 — that extra whole carries into the units: 5 + 1.2 = 6.2.
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.
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).3.4 + 2.8 = ?
5.6 + 1.3 = ?
2.7 + 4.5 = ?
0.8 + 0.6 = ?
12.5 + 3.7 = ?
6.4 + 1.9 = ?
4.8 + 5.6 = ?
9.3 + 0.9 = ?
13.6 + 8.7 = ?
7.5 + 2.5 = ?
24.8 + 15.6 = ?
7.4 + 2.9 = ?