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Step-by-step practice: Maths · Level 3Chapter 9 · Length and mass
Step-by-step Maths Level 3 · Length and mass · Lesson 9.5

Step-by-step Maths Level 3: Adding and subtracting lengths and masses

Part of the Step-by-step Maths Level 3 length and mass chapter. Chapter overview · whole chapter on one page

9.5

Adding and subtracting lengths and masses

Level 3
Add and subtract simple lengths and masses, in m and cm, and in kg and g

To add or subtract, change both measures to the smaller unit. Work it out, then change back.

For example, 1 m 20 cm + 45 cm: 120 cm + 45 cm = 165 cm, which is 1 m 65 cm.

Key ideas

  • Change to the smaller unit.
  • Add or take away.
  • Change back to m and cm, or kg and g.
Worked example 9.5.1
Work out 1 m 20 cm + 45 cm.

Method. Change to cm, work it out, then change back.

  1. Change both to the smaller unit. 100 cm make one bigger unit.
    1 m 20 cm = 120 cm, 45 cm = 45 cm
  2. Add the cm. Work with the numbers in cm.
    120 + 45 = 165
  3. Change back to the bigger units. 165 cm is written with 100 cm as one bigger unit.
    165 cm = 1 m 65 cm
  4. Write the answer. This is the answer.
    1 m 65 cm

Check. Check: 1 m 65 cm - 45 cm = 1 m 20 cm.

Watch out. Work in cm to avoid mistakes. Change back at the end.

Worked example 9.5.2
Work out 2 kg 300 g + 1 kg 400 g.

Method. Change to g, work it out, then change back.

  1. Change both to the smaller unit. 1000 g make one bigger unit.
    2 kg 300 g = 2300 g, 1 kg 400 g = 1400 g
  2. Add the g. Work with the numbers in g.
    2300 + 1400 = 3700
  3. Change back to the bigger units. 3700 g is written with 1000 g as one bigger unit.
    3700 g = 3 kg 700 g
  4. Write the answer. This is the answer.
    3 kg 700 g

Check. Check: 3 kg 700 g - 1 kg 400 g = 2 kg 300 g.

Watch out. Work in g to avoid mistakes. Change back at the end.

Worked example 9.5.3
A ribbon is 2 m 50 cm long. 45 is cut off. How much ribbon is left?

Method. Find the operation, then work out what is left.

  1. Find the operation. Words like cut off and used mean take away.
    250 - 45
  2. Work out what is left. Change to the smaller unit, then take away.
    250 - 45 = 205
  3. Write the answer. Change back to the bigger units.
    2 m 5 cm

Check. Check: 2 m 5 cm plus 45 cm is 2 m 50 cm.

Watch out. Use the same unit for both amounts before you take away.

Watch out

Watch out. 100 cm is 1 m, so 165 cm is 1 m 65 cm, not 165 m.

Exam tip

Grown-up tip. Ask: “How many centimetres is that altogether?” before writing the answer in metres.

Exercise 9E  Practice: adding, subtracting and problems

Non-calculator
1
FWork out 4 kg 250 g - 1 kg 150 g.
2
FWork out 3 kg 750 g + 2 kg 450 g.
3
FWork out 3 kg 250 g + 450 g.
4
FWork out 1 kg 100 g + 2 kg 300 g.
5
FWork out 3 m 20 cm - 1 m 40 cm.
6
FWork out 1 kg 600 g + 2 kg 300 g.
7
FWork out 2 m 30 cm - 1 m 55 cm.
8
FWork out 4 m 75 cm - 1 m 25 cm.
9
PSA bag has 3 kg 500 g of rice. 2 kg is used. How much rice is left?
10
PSA ribbon is 4 m long. 288 is cut off. How much ribbon is left?
11
PSA ribbon is 2 m 80 cm long. 108 is cut off. How much ribbon is left?
12
PSA ribbon is 3 m 80 cm long. 325 is cut off. How much ribbon is left?
13
PSA bag has 2 kg 200 g of rice. 1 kg 500 g is used. How much rice is left?
14
PSA ribbon is 3 m 80 cm long. 149 is cut off. How much ribbon is left?
15
PSA ribbon is 2 m 80 cm long. 120 is cut off. How much ribbon is left?
16
PSA ribbon is 4 m long. 196 is cut off. How much ribbon is left?

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