Write the numbers with their decimal points in a column. Tenths sit under tenths, hundredths under hundredths.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
Why alignment matters
Adding 0.3 to 0.05 is not 0.08. Three tenths and five hundredths are different units.
Fill the gaps
Writing 0.3 as 0.30 makes both numbers the same length without changing either.
Estimate first
4.7 + 2.9 is roughly 5 + 3, so about 8. An answer of 0.76 or 76 is visibly wrong.
Align the points, not the ends
Writing 12.4 + 3.75 with the right-hand edges aligned adds tenths to hundredths and gives nonsense. Align the decimal points so like units meet. This is the same principle as columns in whole-number addition, and it is the only thing that can really go wrong.
Pad to equal length
Write 12.40 + 3.75 so both show hundredths. Padding costs nothing, changes no values and removes every temptation to misalign. Do it before you start rather than discovering the gap halfway through.
Whole numbers have a hidden point
15 is 15.00. When a whole number appears in a decimal calculation, write the point and the zeros explicitly. Most alignment errors in mixed calculations start with a whole number written without its decimal places.
Estimate to whole numbers
12.4 + 3.75 is roughly 12 + 4 = 16. The exact answer 16.15 fits. Since alignment errors shift the answer by a factor of ten, even the crudest estimate exposes them.
Step 2: Try It Yourself
Tap and try it out.
30/100 = 0.30 = 0.3
5/100 = 0.05
0.3 covers more squares than 0.05, so it is greater.
Step 3: Watch an Example
One step at a time.
Watch Ana Add 4.7 and 2.85
Ana adds two decimals of different lengths.
- Step 1
She estimates first: about 5 plus about 3, so around 8.
Step 4: Your Turn
Practice makes it stick.
The Shopping
Problem 1 of 2
£3.45 and £2.30 together, in pounds?
The Change
Problem 2 of 2
£10 minus £6.40, in pounds?
Point Under Point
1 of 4
2.4 + 1.35 = ?
2 of 4
5.6 − 2.45 = ?
3 of 4
0.3 + 0.05 = ?
4 of 4
7 − 3.25 = ?
Step 5: Quick Check
Show what you know.
Question 1 of 1
6.4 + 2.75 = ?
What You Learned
- Line up the decimal points, not the last digits.
- Fill short numbers with zeros so both have the same length.
- Estimate first, so a misplaced point is obvious.