Lesson 6 of 8 · Grades 3 to 5
Telling time and elapsed time
An analog clock shows hours and minutes on one face. Reading it is one skill; working out how much time passes between two times is another. This lesson practises both with a clock face and a time line.
The two hands
An analog clock has one face with two hands. The short hand shows the hour and the long hand shows the minutes. The long hand goes all the way around the face in 60 minutes, which is one hour. Every number on the face is five minutes further on, so the 1 is 5 minutes, the 2 is 10 minutes, the 3 is 15 minutes, and the 6, at the bottom, is 30 minutes.
Read the hour first, then the minutes. At 7:20, the short hand has just left the 7 and is on its way to the 8, and the long hand points at the 4, which is 20 minutes past the hour. Say it as "seven twenty" or "twenty minutes past seven".
Minutes past and minutes to
The first half of an hour counts minutes past: 7:10, 7:20, 7:30. The second half can count towards the next hour. At 7:40 the minute hand points at the 8, which is 20 minutes before 8:00, so 7:40 is also "twenty minutes to eight". Both readings name the same time. Counting to the next hour is useful when you work out elapsed time, because the next whole hour is an easy place to stop.
Clocks show time twice a day. One full day has 24 hours, and the clock face runs through 12 of them twice. That is why times carry a.m. or p.m.: a.m. runs from midnight to noon, and p.m. runs from noon to midnight. A plane at 7:20 a.m. leaves in the morning, and a film at 7:20 p.m. starts in the evening.
Elapsed time: count on
Elapsed time is the amount of time between two moments. To find it, count on from the start time to the end time, and stop at the next whole hour on the way. A football match starts at 2:45 and ends at 4:20. Count 15 minutes to 3:00, then 60 minutes to 4:00, then 20 minutes to 4:20. The three jumps add up to 15 + 60 + 20 = 95 minutes, which is 1 hour 35 minutes.
Going through an hour
When a start time and an end time sit on opposite sides of an hour, split the count at the hour. From 7:35 to 8:00 is 25 minutes. From 9:40 to 10:25 is 20 minutes to 10:00 and 25 minutes after it, so 45 minutes in total. The same method works when the count crosses noon or midnight: count the minutes to the next hour, the whole hours after that, and the minutes left over at the end.
To find an end time, start at the given time and add the elapsed time in the same way. A class that starts at 8:30 and lasts 45 minutes ends at 9:15, because 30 minutes take it to 9:00 and 15 more minutes take it to 9:15. Add minutes to minutes and keep the hour in the right place.
Watch out for this
A clock column does not hold numbers past 59. If you add 20 minutes to 1:50, do not write 1:70. Count to the next hour instead: 10 minutes take you to 2:00, and the last 10 minutes take you to 2:10. Time borrows in sixties, not in tens, because an hour holds 60 minutes and a minute holds 60 seconds.
Worked example
A football match starts at 2:45 and ends at 4:20. How long is the match?
Step 1: count from the start to the next hour. From 2:45 to 3:00 is 15 minutes.
Step 2: count the whole hours. From 3:00 to 4:00 is 60 minutes.
Step 3: count the last part. From 4:00 to 4:20 is 20 minutes.
Step 4: add the parts. 15 + 60 + 20 = 95 minutes, which is 1 hour 35 minutes.
Answer: 95 minutes, or 1 hour 35 minutes.
Remember
- The short hand shows hours and the long hand shows minutes.
- Each number on the face is five minutes.
- Count elapsed time on in jumps, stopping at the next whole hour.
- An hour holds 60 minutes and a minute holds 60 seconds.
Try it
Answer the five questions below. Work them out first, then open each answer to check.
-
How many minutes are in 2 hours?
Show answer
120 minutes. Each hour has 60 minutes, and 2 × 60 = 120 min.
-
A film starts at 4:15 and lasts 30 minutes. What time does it end?
Show answer
4:45. Thirty minutes after 4:15 is 4:45.
-
How many minutes are from 2:50 to 3:10?
Show answer
20 minutes. Count 10 minutes to 3:00, then 10 more minutes.
-
How many seconds are in 3 minutes?
Show answer
180 seconds. 3 × 60 = 180 s.
-
A train leaves at 9:40 and arrives at 10:25. How long is the trip?
Show answer
45 minutes. Count 20 minutes to 10:00, then 25 more minutes.
Puzzle · Clock puzzle
Puzzle time
The clock shows 7:20. What time will it be in 45 minutes?
Show a hint
Add 40 minutes to 7:20 to reach 8:00, then add the last 5 minutes.
Show the answer
Answer: 8:05. 7:20 plus 40 minutes is 8:00, and 5 more minutes makes 8:05.