Length Measurements
Primary 4 · Mathematics
Length Measurements
Lesson 4 of 5
25 min
Let's begin
Ishimwe is making a skipping rope. He ties together two pieces of sisal rope: one is 3 m 65 cm long and the other is 2 m 50 cm. How long is the new rope?
Write the lengths in two columns, m and cm. Add the centimetres first, then the metres. Remember that 100 cm = 1 m: if the centimetres make 100 or more, change 100 cm into 1 m and carry it.
| m | cm | |
| carry | 1 | |
| 3 | 65 | |
| + | 2 | 50 |
| 6 | 15 |
Ishimwe's rope is 6 m 15 cm long.
A piece of cloth is 5 m 20 cm long. The tailor cuts off 2 m 45 cm. How much is left? We cannot take 45 cm from 20 cm, so we borrow 1 m = 100 cm: 5 m 20 cm becomes 4 m 120 cm.
| m | cm | |
| regrouped | 4 | 120 |
| − | 2 | 45 |
| 2 | 75 |
There are 2 m 75 cm of cloth left.
It works the same way, but 1 000 m = 1 km. A moto rider goes 2 km 400 m to the market and then 1 km 800 m to the school. 400 m + 800 m = 1 200 m = 1 km 200 m, so the total is 2 km + 1 km + 1 km 200 m = 4 km 200 m.
Another way: change everything to one unit first. 2 km 400 m = 2 400 m and 1 km 800 m = 1 800 m. 2 400 + 1 800 = 4 200 m = 4 km 200 m.
Tip
Know your hundred and your thousand. In m and cm, carry or borrow 100 (100 cm = 1 m). In km and m, carry or borrow 1 000 (1 000 m = 1 km).
Work each one in your exercise book first. Then tap Show answer to check.
4 m 30 cm + 2 m 45 cm
Answer: 6 m 75 cm
1 m 70 cm + 2 m 60 cm
Answer: 4 m 30 cm (70 + 60 = 130 cm = 1 m 30 cm)
6 m 15 cm − 3 m 40 cm
Answer: 2 m 75 cm
5 km − 2 km 300 m
Answer: 2 km 700 m (5 000 m − 2 300 m = 2 700 m)
Umutoni has a rope 3 m 50 cm long. She cuts off 1 m 75 cm. How much rope is left?
Answer: 1 m 75 cm
What you learned today
Sources and credits
The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.
Quick check
2 m 80 cm + 1 m 40 cm = ?
