3B · Chapter 11 · Lesson 2
Subtracting from Meters
Regroup one meter as 100 centimeters when subtracting mixed lengths.
Today's mathematical question
Subtract 78 cm from 4 m 65 cm. Give mixed units.
Learning target
Regroup one meter as 100 centimeters when subtracting mixed lengths.
Before starting: Prior lesson: Meters and Centimeters. Try the warm-up independently.
Prerequisite · 3 questions
Check earlier skills first
These are earlier ideas you need for today; revisit them if they are difficult.
- P1
Convert 3 m 55 cm into cm.
Reveal explanation
Answer: 355 cm
3 m = 300 cm; add 55 to make 355 cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - P2
Express 465 cm as m and cm.
Reveal explanation
Answer: 4 m 65 cm
Partition 465 into 4 full groups of 100 plus 65.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - P3
Convert 7 m 75 cm into cm.
Reveal explanation
Answer: 775 cm
7 m = 700 cm; add 75 to make 775 cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity.
Learn and understand
Subtracting mixed meters and centimeters may require exchanging one meter for 100 centimeters. Imagine 3 m 20 cm minus 75 cm: there are not enough centimeters in the second part, so rename 3 m 20 cm as 2 m 120 cm. Then subtract 75 cm to find 2 m 45 cm. To subtract 70 cm from 3 m 20 cm, exchange one meter for 100 cm: 2 m 120 cm − 70 cm = 2 m 50 cm. The actual length has not changed during the exchange.
Represent it three ways
- Concrete: Build equal pieces, measures, counters or shapes with real objects when useful.
- Pictorial: Write meters and centimeters in separate columns; explicitly show the one-meter exchange before subtracting.
- Abstract: Write the matching number sentence, calculation, comparison or mathematical description. Explain each number's unit.
Important vocabulary
- regroup
- rename equivalent quantities to make subtraction possible
- exchange
- trade one large unit for an equal collection of small units
- difference
- result of subtraction
Fully worked examples and checks
Try each new understanding check before revealing the explanation.
Worked example 1
Explore, calculate, verify
Subtract 78 cm from 4 m 65 cm. Give mixed units.
- Identify the whole, the units, and what the question asks.
- Exchange one m for 100 cm, then subtract: 465 − 78 = 387 cm. Regroup the result into m and cm.
- Check that the answer uses the right units and makes sense.
Answer: 3 m 87 cm
Check your understanding 1
Subtract 82 cm from 2 m 50 cm. Give mixed units.
Reveal answer and explanation
1 m 68 cm — Exchange one m for 100 cm, then subtract: 250 − 82 = 168 cm. Regroup the result into m and cm.
Worked example 2
Explore, calculate, verify
Subtract 26 cm from 8 m 25 cm. Give mixed units.
- Identify the whole, the units, and what the question asks.
- Exchange one m for 100 cm, then subtract: 825 − 26 = 799 cm. Regroup the result into m and cm.
- Check that the answer uses the right units and makes sense.
Answer: 7 m 99 cm
Check your understanding 2
Subtract 92 cm from 3 m 65 cm. Give mixed units.
Reveal answer and explanation
2 m 73 cm — Exchange one m for 100 cm, then subtract: 365 − 92 = 273 cm. Regroup the result into m and cm.
Worked example 3
Explore, calculate, verify
Subtract 36 cm from 4 m 25 cm. Give mixed units.
- Identify the whole, the units, and what the question asks.
- Exchange one m for 100 cm, then subtract: 425 − 36 = 389 cm. Regroup the result into m and cm.
- Check that the answer uses the right units and makes sense.
Answer: 3 m 89 cm
Check your understanding 3
Subtract 53 cm from 7 m 50 cm. Give mixed units.
Reveal answer and explanation
6 m 97 cm — Exchange one m for 100 cm, then subtract: 750 − 53 = 697 cm. Regroup the result into m and cm.
Guided · 5 questions
Guided practice
Keep the model and earlier examples visible while practising.
- G1
Subtract 79 cm from 3 m 20 cm. Give mixed units.
Reveal explanation
Answer: 2 m 41 cm
Exchange one m for 100 cm, then subtract: 320 − 79 = 241 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - G2
Subtract 80 cm from 5 m 40 cm. Give mixed units.
Reveal explanation
Answer: 4 m 60 cm
Exchange one m for 100 cm, then subtract: 540 − 80 = 460 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - G3
Subtract 73 cm from 6 m 20 cm. Give mixed units.
Reveal explanation
Answer: 5 m 47 cm
Exchange one m for 100 cm, then subtract: 620 − 73 = 547 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - G4
Subtract 85 cm from 7 m 55 cm. Give mixed units.
Reveal explanation
Answer: 6 m 70 cm
Exchange one m for 100 cm, then subtract: 755 − 85 = 670 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - G5
Subtract 96 cm from 7 m 80 cm. Give mixed units.
Reveal explanation
Answer: 6 m 84 cm
Exchange one m for 100 cm, then subtract: 780 − 96 = 684 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity.
Independent · 8 questions
Independent practice
Work these without hints and explain the method used. Write answers on paper.
- I1
Subtract 91 cm from 6 m 80 cm. Give mixed units.
Reveal explanation
Answer: 5 m 89 cm
Exchange one m for 100 cm, then subtract: 680 − 91 = 589 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I2
Subtract 34 cm from 5 m 20 cm. Give mixed units.
Reveal explanation
Answer: 4 m 86 cm
Exchange one m for 100 cm, then subtract: 520 − 34 = 486 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I3
Subtract 96 cm from 2 m 25 cm. Give mixed units.
Reveal explanation
Answer: 1 m 29 cm
Exchange one m for 100 cm, then subtract: 225 − 96 = 129 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I4
Subtract 55 cm from 8 m 50 cm. Give mixed units.
Reveal explanation
Answer: 7 m 95 cm
Exchange one m for 100 cm, then subtract: 850 − 55 = 795 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I5
Subtract 62 cm from 3 m 50 cm. Give mixed units.
Reveal explanation
Answer: 2 m 88 cm
Exchange one m for 100 cm, then subtract: 350 − 62 = 288 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I6
Subtract 84 cm from 4 m 40 cm. Give mixed units.
Reveal explanation
Answer: 3 m 56 cm
Exchange one m for 100 cm, then subtract: 440 − 84 = 356 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I7
Subtract 67 cm from 2 m 25 cm. Give mixed units.
Reveal explanation
Answer: 1 m 58 cm
Exchange one m for 100 cm, then subtract: 225 − 67 = 158 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - I8
Subtract 37 cm from 6 m 20 cm. Give mixed units.
Reveal explanation
Answer: 5 m 83 cm
Exchange one m for 100 cm, then subtract: 620 − 37 = 583 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity.
Challenge · 2 questions
Challenge and error analysis
Go beyond arithmetic: justify a claim, correct reasoning, or connect two representations.
- C1
A student claims 7 m 40 cm means 740 m. What unit should the total use?
Reveal explanation
Answer: 740 cm
There are 100 centimeters in one meter, so 7 m 40 cm equals 740 cm, not meters.
- C2
Explain how to subtract 70 cm from 7 m 20 cm without writing a negative number of centimeters.
Reveal explanation
Answer: 6 m 50 cm
Exchange 1 m for 100 cm: 6 m 120 cm − 70 cm = 6 m 50 cm.
Exit · 3 questions
Exit check
Solve independently, then return on another day to check retention.
- E1
Subtract 93 cm from 6 m 80 cm. Give mixed units.
Reveal explanation
Answer: 5 m 87 cm
Exchange one m for 100 cm, then subtract: 680 − 93 = 587 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - E2
Subtract 84 cm from 8 m 50 cm. Give mixed units.
Reveal explanation
Answer: 7 m 66 cm
Exchange one m for 100 cm, then subtract: 850 − 84 = 766 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity. - E3
Subtract 98 cm from 5 m 80 cm. Give mixed units.
Reveal explanation
Answer: 4 m 82 cm
Exchange one m for 100 cm, then subtract: 580 − 98 = 482 cm. Regroup the result into m and cm.
One larger unit is exchanged into an equal number of smaller units without changing the quantity.