Decimal Numbers
Primary 4 · Mathematics
Decimal Numbers
Lesson 4 of 4
30 min
Let's begin
Keza's mother is a tailor in Nyamirambo. For a new dress she buys 2.75 m of blue kitenge and 1.5 m of yellow kitenge. How many metres of cloth does she buy altogether?
Adding decimals works just like adding whole numbers, with one extra rule: line up the decimal points. Then tenths are under tenths and hundredths are under hundredths. Fill any empty place with a 0.
| Ones | . | Tenths | Hundredths | |
| carry | 1 | |||
| 2 | . | 7 | 5 | |
| + | 1 | . | 5 | 0 |
| 4 | . | 2 | 5 |
Keza's mother buys 4.25 m of cloth.
Mugisha has 5 litres of water in a jerrycan and pours out 1.35 litres. How much is left? Write 5 as 5.00 so both numbers have two decimal places, then subtract, borrowing as with whole numbers.
| Ones | . | Tenths | Hundredths | |
| regrouped | 4 | 9 | 10 | |
| . | ||||
| − | 1 | . | 3 | 5 |
| 3 | . | 6 | 5 |
There are 3.65 litres left. Check: 3.65 + 1.35 = 5.00.
Estimate first, so you can tell if an answer is sensible. 2.75 + 1.5 is about 3 + 1.5 = 4.5, and our answer 4.25 is close. If you got 2.90, you would know that something went wrong: the points were not lined up!
Tip
Point under point. Before you add or subtract decimals, stack the decimal points in a straight line, fill the gaps with zeros, and keep the point in the same place in your answer.
Work each one in your exercise book first. Then tap Show answer to check.
4.6 + 2.3
Answer: 6.9
1.75 + 3.48
Answer: 5.23
8.5 − 2.7
Answer: 5.8
10 − 4.25
Answer: 5.75 (write 10 as 10.00 first)
In a relay, Ishimwe runs 1.2 km, Mutesi runs 0.85 km and Habimana runs 0.6 km. How far do they run altogether?
Answer: 2.65 km
What you learned today
Sources and credits
Siyavula writes decimals with a comma (0,3); this lesson uses the decimal point (0.3). The stories, Rwandan worked examples and practice questions were written by Ganzaa.
Quick check
