Concept 6 of 8
Add & Subtract Decimals
Today's big idea
Line up the decimal points vertically, then add/subtract column by column.
Line up the decimal points, not the last digits, so tenths meet tenths.
How it works
👀 What's going on?
Two decimals go wrong the moment you line up their last digits. Money and metres do not behave that way. The point is the thing that has to line up.
💡 Here's the trick
Look at the stack above. The points sit in one straight column, and every number is padded with zeros to the same length. After that it is ordinary column adding.
🪄 How to do it
- 1.Copy the numbers into a column like the stack above.
- 2.Pad each one with zeros so the lengths match.
- 3.Add or subtract by column, then drop the point straight down.
✍️ Try it
Work out 9.6 + 0.475. Then work out 20 minus 6.85.
Padding gives 9.600 plus 0.475, which is 10.075. The second becomes 20.00 minus 6.85, and that leaves 13.15.
⚠️ Watch out
You might be thinking the last digits should line up, the way they do for whole numbers. Here that adds tenths to hundredths. Lining 6.2 under 1.45 by the last digit gives 2.07, not 7.65.
🌟 Why this matters
Shop bills, height charts and race times are all added this way. One point out of place turns rupees into paise.
Exam tip: Pad with zeros first, every single time, even when it looks unnecessary.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“Find: 12.4 + 3.075.”
- 1
Stack the decimal points
Line up . right under . 📏
- 2
Pad with zeros
12.4 → 12.400. Now both have 3 places.
- 3
Add column by column
12.400 + 3.075
- 4
Drop the point ✓
= 15.475
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Always line up the decimal POINT (not the right edge). Pad with zeros if needed.
EXPRESSION · ROUND 1 OF 3
6.3 + 1.45
Quick check
✅ VERIFICATION
What is 8.3 − 4.16?
⚡ Finish the activity above first to answer.
Keep going!
Up next: × & ÷ Decimals
Slide the decimal point.