Week 6 · Time, scales and measurement problems · 3 lessons
1
Reading the clock and elapsed time
Discover Mathematics P4: Term 1, Unit 3, Lessons 5–9
GoalRead and write times, change between units of time, and find how much time has passed.
New words
elapsed time · the time that passes from the start of something to its enda.m. / p.m. · a.m. is from midnight to noon; p.m. is from noon to midnightsecond (s) · a very short time; 60 seconds make 1 minute
Explanation
We tell the time with hours and minutes, and we add a.m. for times before noon and p.m. for times after noon. The units of time are linked like this: 1 minute = 60 seconds, 1 hour = 60 minutes, 1 day = 24 hours and 1 week = 7 days. To change hours to minutes, multiply by 60: 2 h 15 min = 2 × 60 + 15 = 135 min. To find elapsed time, start at the first time and jump to the next full hour, then add whole hours, then add the last minutes. A football match starts at 4:15 p.m. and ends at 6:05 p.m. From 4:15 to 5:00 is 45 min, from 5:00 to 6:00 is 60 min, and from 6:00 to 6:05 is 5 min. The total is 45 + 60 + 5 = 110 min, which is 1 h 50 min.
Worked examples
The match starts at 4:15 p.m. and ends at 6:05 p.m. The jumps are 45 min to 5:00, 60 min to 6:00 and 5 min to 6:05. In all 45 + 60 + 5 = 110 min = 1 h 50 min.
From
To
Work
2 h 15 min
135 min
2 × 60 + 15
300 s
5 min
300 ÷ 60
3 days
72 h
3 × 24
Change units of time. 2 h 15 min = 2 × 60 + 15 = 135 min. 300 s = 300 ÷ 60 = 5 min. 3 days = 3 × 24 = 72 h. 2 weeks = 2 × 7 = 14 days.
Salma leaves home at 7:40 a.m. and reaches school at 8:25 a.m. From 7:40 to 8:00 is 20 min and from 8:00 to 8:25 is 25 min. The trip takes 20 + 25 = 45 min.
Class activity
Daily timetable. Write the times of five things you do every day, such as waking up, leaving for school, lunch, homework and bedtime. Use a.m. and p.m. Then find how long it is from waking up to leaving for school, and from lunch to bedtime.
Practice
1
Change 3 h 20 min to minutes.
3 × 60 + 20 = 200 min.
2
Change 420 seconds to minutes.
420 ÷ 60 = 7 min.
3
A film starts at 6:30 p.m. and ends at 8:10 p.m. How long is it?
6:30 to 7:00 is 30 min, 7:00 to 8:00 is 60 min, 8:00 to 8:10 is 10 min: 100 min = 1 h 40 min.
4
Youssef starts homework at 4:50 p.m. and finishes after 55 minutes. What time does he finish?
