Time
Primary 4 · Mathematics
Time
Lesson 3 of 5
20 min
Let's begin
Niyonzima's football practice lasts 90 minutes. "That is one and a half hours," says the coach.
Niyonzima's little sister asks, "How many seconds is that?" Niyonzima has no idea!
Today you will change seconds, minutes, hours, days, weeks, months and years from one unit to another.
| This | is the same as |
|---|---|
| 1 minute (min) | 60 seconds (s) |
| 1 hour (h) | 60 minutes |
| 1 day | 24 hours |
| 1 week | 7 days |
| 1 year | 12 months |
| 1 year | 365 days (366 in a leap year) |
| 1 decade | 10 years |
| 1 century | 100 years |
Careful! Time does not count in tens like metres and grams. 1 hour is 60 minutes, not 100.
Worked example 1
3 h = ? min. 3 × 60 = 180 min.
Worked example 2
4 min = ? s. 4 × 60 = 240 s.
Worked example 3
2 days = ? h. 2 × 24 = 48 h.
Worked example 4
2 h 15 min = ? min. 2 × 60 = 120, then 120 + 15 = 135 min.
Worked example 5
5 weeks = ? days. 5 × 7 = 35 days.
Worked example 6
150 min = ? h ? min.
Step 1 How many 60s fit in 150? 60 × 2 = 120, so 2 hours.
Step 2 150 − 120 = 30 minutes are left over.
Step 3 So 150 min = 2 h 30 min.
Worked example 7
72 h = ? days. 72 ÷ 24 = 3 days.
So Niyonzima's 90 minutes is 60 + 30 = 1 h 30 min, which is 1 12 h. In seconds it is 90 × 60 = 5 400 s!
Tip
Time counts in 60s, not 10s. 60 seconds make a minute and 60 minutes make an hour. Then 24 hours make a day and 7 days make a week.
Work each one in your exercise book first. Then tap Show answer to check.
5 h = ___ min
Answer: 300 min, because 5 × 60 = 300.
3 min 20 s = ___ s
Answer: 200 s. 3 × 60 = 180, and 180 + 20 = 200.
200 min = ___ h ___ min
Answer: 3 h 20 min. 3 × 60 = 180, and 200 − 180 = 20.
3 weeks = ___ days
Answer: 21 days, because 3 × 7 = 21.
96 h = ___ days
Answer: 4 days, because 96 ÷ 24 = 4.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
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