Time
Primary 4 · Mathematics
Time
Lesson 5 of 5
20 min
Let's begin
Habimana takes the bus from Kigali to Huye to visit his grandmother. The bus leaves Nyabugogo at 9:45 a.m. It arrives in Huye at 12:10 p.m.
How long was the journey? Today you will find how long things take, and when they start and end.
To find how long something takes, start at the start time. Jump to the next o'clock, then jump whole hours, then the minutes that are left.
Worked example 1
From 9:45 to 10:00 is 15 min. From 10:00 to 12:00 is 2 h. From 12:00 to 12:10 is 10 min. Altogether: 15 min + 2 h + 10 min = 2 h 25 min.
Worked example 2
A film starts at 7:50 p.m. and lasts 1 h 45 min. When does it end?
Step 1 Add the hour: 7:50 p.m. + 1 h = 8:50 p.m.
Step 2 Add 10 min to reach 9:00 p.m. There are 45 − 10 = 35 min left.
Step 3 9:00 p.m. + 35 min = 9:35 p.m.
Worked example 3
Keza must be at school at 7:30 a.m. Her walk takes 40 minutes. When must she leave home?
Step 1 Jump back 30 min: 7:30 to 7:00.
Step 2 Jump back 10 more minutes: 7:00 to 6:50 a.m.
Worked example 4
1 h 45 min + 2 h 30 min. Add the minutes: 45 + 30 = 75 min = 1 h 15 min. Add the hours: 1 + 2 + 1 = 4 h. The answer is 4 h 15 min.
Worked example 5
3 h 10 min − 1 h 40 min. 10 min take away 40 min? Not enough. Break 1 h into 60 min: 60 + 10 = 70 min, and 2 h are left. 70 − 40 = 30 min, and 2 − 1 = 1 h. The answer is 1 h 30 min.
Careful! When you break an hour, you get 60 minutes, not 100. Do not subtract times like ordinary numbers: 12:10 − 9:45 is NOT 2 h 65 min.
Tip
Jump to the next o'clock. From the start time, jump to the next full hour, then jump whole hours, then the minutes left. Add up your jumps.
Work each one in your exercise book first. Then tap Show answer to check.
A lesson starts at 10:15 a.m. and ends at 10:55 a.m. How long is it?
Answer: 40 minutes. 55 − 15 = 40.
Mama goes to the market at 8:40 a.m. and comes home at 11:15 a.m. How long was she away?
Answer: 2 h 35 min. 8:40 to 9:00 is 20 min, 9:00 to 11:00 is 2 h, 11:00 to 11:15 is 15 min.
A cake goes into the oven at 2:35 p.m. It bakes for 50 minutes. When is it ready?
Answer: 3:25 p.m. 2:35 to 3:00 is 25 min, and 25 more minutes makes 3:25.
2 h 50 min + 1 h 25 min
Answer: 4 h 15 min. 50 + 25 = 75 min = 1 h 15 min. Then 2 + 1 + 1 = 4 h.
Umuganda goes from 8:00 a.m. to 11:00 a.m. How many minutes is that?
Answer: 180 minutes. It is 3 hours, and 3 × 60 = 180.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
Quick check
