Mass Measurements
Primary 4 · Mathematics
Mass Measurements
Lesson 4 of 4
20 min
Let's begin
Umutoni sells tomatoes and beans at Kimironko market. One customer asks for 3 kg of tomatoes. Another wants only half a kilogram. A restaurant wants beans packed in 500 g bags.
Umutoni must weigh, compare, add, take away and work out prices. Today you will solve problems like hers.
Worked example 1
Order from lightest to heaviest: 1.2 kg, 950 g, 1 kg 50 g, 1 500 g.
Step 1 Change to grams: 1.2 kg = 1 200 g and 1 kg 50 g = 1 050 g.
Step 2 Compare 1 200, 950, 1 050 and 1 500.
Step 3 Order: 950 g, 1 kg 50 g, 1.2 kg, 1 500 g.
Worked example 2
Tomatoes cost 800 Frw per kg. What do 3 kg cost? What does half a kilogram cost?
3 × 800 = 2 400 Frw. Half a kilogram costs half of 800 Frw: 800 ÷ 2 = 400 Frw. So 3 kg 500 g costs 2 400 + 400 = 2 800 Frw.
Worked example 3
Umutoni packs 12 kg of beans into 500 g bags. How many bags does she fill?
Step 1 Same unit: 12 kg = 12 000 g.
Step 2 Divide: 12 000 ÷ 500 = 24. She fills 24 bags.
Worked example 4
A pickup can carry 1 t. It has 16 sacks of 50 kg on it. How many more kilograms can it carry?
Step 1 Mass on the pickup: 16 × 50 = 800 kg.
Step 2 1 t = 1 000 kg, so 1 000 − 800 = 200 kg more.
Always look back. A hen of 2 t? No, a hen is about 2 kg. A lorry of 5 g? No, 5 t. If the answer is silly, check your units.
Tip
Change, choose, check. Change every mass to one unit. Choose the operation. Check that the answer makes sense in real life.
Work each one in your exercise book first. Then tap Show answer to check.
Which is heavier: 1.5 kg of sugar or 1 400 g of rice?
Answer: The sugar. 1.5 kg = 1 500 g, which is more than 1 400 g.
Beans cost 1 000 Frw per kg. How much do 4 kg cost? How much does half a kilogram cost?
Answer: 4 kg cost 4 000 Frw. Half a kilogram costs 500 Frw.
How many 250 g packets can be filled from 5 kg of groundnuts?
Answer: 20 packets. 5 kg = 5 000 g, and 5 000 ÷ 250 = 20.
Mugisha's mass is 32 kg. His little sister's mass is 18.5 kg. How much heavier is Mugisha?
Answer: 13.5 kg. 32.0 − 18.5 = 13.5.
A lorry carries 4 t of cement. It unloads 1 t 250 kg at a school. How much cement is still on the lorry?
Answer: 2 t 750 kg. 4 000 kg − 1 250 kg = 2 750 kg.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
Quick check
Put these in order, lightest first.
Tap the cards in the right order.
