Discover Mathematics P4: Term 1, Unit 3, Lessons 5–9
GoalSolve measurement stories that need multiplying or dividing, using the right units.
New words
equal parts · pieces that are all the same sizesame unit · before we calculate we change all amounts to one unitmultiply / divide · to find the total of equal groups / to share equally
Explanation
Many measurement stories use equal parts. If there are equal pieces, multiply to find the total. If we cut or share a total into equal pieces, divide. Always follow three steps. Step 1: change all the measures to the same small unit, such as cm, g or mL. Step 2: multiply or divide. Step 3: change the answer back to a sensible unit and write it with its unit. For example, a rope 4 m long is cut into pieces of 50 cm. 4 m = 400 cm and 400 ÷ 50 = 8, so there are 8 pieces. Six bottles of 250 mL hold 6 × 250 = 1,500 mL = 1 L 500 mL. Always ask: does the size of my answer make sense?
Worked examples
A rope 4 m long is cut into pieces of 50 cm. Change 4 m to 400 cm. Then 400 ÷ 50 = 8 pieces. The bar model shows 8 equal parts of 50 cm.
Step
Work
1
3 km 500 m = 3,500 m
2
3,500 × 4 = 14,000 m
3
14,000 m = 14 km
Karim rides his bicycle 3 km 500 m every day for 4 days. 3 km 500 m = 3,500 m. 3,500 × 4 = 14,000 m = 14 km in all.
A jug holds 2 L 400 mL of juice. It is shared equally among 8 cups. 2 L 400 mL = 2,400 mL and 2,400 ÷ 8 = 300 mL. Each cup gets 300 mL. A laps story: one lap takes 4 min 30 s = 270 s. Six laps take 270 × 6 = 1,620 s = 27 min.
Class activity
Measure a recipe. Pick a simple recipe at home, for example a drink that needs 250 mL of milk for 1 person. Find how much milk you need for 6 people and write it in mL and in L. Then do the same with 2 spoonfuls of sugar of 15 g each.
Practice
1
Six bottles hold 250 mL each. How many L and mL is that?
6 × 250 = 1,500 mL = 1 L 500 mL.
2
A ribbon is 3 m long. How many pieces of 30 cm can be cut from it?
3 m = 300 cm; 300 ÷ 30 = 10 pieces.
3
Three bags of rice are 2 kg 500 g each. What is the total mass?
2,500 × 3 = 7,500 g = 7 kg 500 g.
4
A film is 90 minutes long. Nour watches it 4 times. How many hours is that?
90 × 4 = 360 min = 6 hours.
5
A tank has 12 L of water and each bottle holds 750 mL. Ahmed says he can fill 16 bottles. Is he right?
12 L = 12,000 mL and 12,000 ÷ 750 = 16. Yes, he is right.
