Concept 4 of 7
Mixed Units: Add and Subtract Smartly
Today's big idea
Convert to the smaller unit, do the maths, then convert back.
Borrowing crosses units, so 1 m lends 100 cm and never 10 cm.
How it works
👀 What's going on?
Shop labels love two units at once: 3 kg 750 g, or 2 m 45 cm. Adding those straight down goes wrong the moment the small part spills over.
💡 Here's the trick
Look at the sum above. Both masses became grams before anything was added. In one unit there are no borrowing surprises, and you convert back only at the end.
🪄 How to do it
- 1.Copy the sum above and pick the smaller unit.
- 2.Rewrite both measurements in that unit.
- 3.Add or subtract, then convert back.
✍️ Try it
Add 2 m 65 cm and 1 m 70 cm. Then: take 1 m 55 cm from 4 m 10 cm.
In cm the first is 265 + 170 = 435 cm, which is 4 m 35 cm. The second is 410 − 155 = 255 cm, which is 2 m 55 cm.
⚠️ Watch out
You might be thinking you can borrow 10 from the metres, the way you do with plain digits. It does not work that way. Here 1 m lends 100 cm.
🌟 Why this matters
Cutting cloth, stacking parcels and measuring a shelf all mix two units on one line. Working in a single unit turns the whole job into ordinary adding.
Exam tip: Never leave 1,130 g as your final line when the question wants kg and g.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“Add: 3 kg 750 g + 2 kg 400 g.”
- 1
Convert each to grams
3 kg 750 g = 3,750 g. 2 kg 400 g = 2,400 g. 🧮
- 2
Add
3,750 + 2,400 = 6,150 g
- 3
Re-group into kg + g
6,150 g = 6 kg + 150 g.
- 4
Answer ✓
6 kg 150 g
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Convert to the SMALLER unit on BOTH sides, compute, then convert back. One tap at a time.
MIXED UNITS · ROUND 1 OF 3
2 m 30 cm + 1 m 80 cm
Quick check
✅ VERIFICATION
Add: 1 kg 850 g + 2 kg 300 g.
⚡ Finish the activity above first to answer.
Keep going!
Up next: Decimals in Measurement
3 m 5 cm = 3.05 m, count the zeros.