Table of contents  
Clocks  
Telling the Time  
Using A.M. and P.M. Time  
24Hour Time Notation  
Conversion of Time  
Operations on Measures of Time  
Elapsed Time 
Look at the following clocks.
Here are some units for measuring time:
Edurev Tips:
 When the minute hand is before 6, we read it as past.
 When the minutehand is after 6, we read it as to...
The time shown on the clock given alongside is 8:25, but we are not able to determine whether it is 8:25 in the morning or evening.
A day has 24 hours, so in a 12hour clock (that we use) the hour hand goes around the clock two times and shows similar time twice a day.To differentiate this, we use a.m. (ante meridian) to show time from after midnight to before noon and p.m. (post meridian) to show time from afternoon to midnight.
So, 8:25 in the morning is written as 8:25 a.m. and the same time at night is written as 8:25 p.m.
Edurev Tips: A day begins at 12 midnight and ends at 12 midnight of the following day.
We have already studied the 24hour clock in Class 3. The 24hour time notation is represented by a ‘24hour clock’.
Here, the time from 12 midnight to 12 midnight (1 day) is given as 0000 hours to 2400 hours, where the first two digits refer to hours and the last two digits to minutes.
The table given below shows the 24hour time equivalent to12hour time:
Thus,
Look at the following clocks and tell what time they show.
Edurev Tips:
1. Addition
Example 1: Add 6 hours 45 minutes and 11 hours 39 minutes.
Ans:
Thus, 6 hours 45 minutes + 11 hours 39 minutes = 18 hours 24 minutes.
Think:
45 min + 39 min = 84 min
= 60 min + 24 min
= 1 h + 24 min
2. Subtraction
Example 2: Raju completes a painting in 6 hours 26 minutes and Rohit completes it in 4 hours 45 minutes. How much more time does Raju take to complete the painting?
Ans: You have to subtract 4 hours 45 minutes from 6 hours 26 minutes to find the required time.
Thus, Raju takes 1 hour 41 minutes more to complete the painting.
Think:
45 min cannot be subtracted from 26 min.
Borrow, 1 h = 60 min from 6 hours.
60 min + 26 min = 86 min
86 min – 45 min = 41 min
5 h – 4 h = 1 h
Using the 24hour time notation and our knowledge of addition and subtraction of time, we can easily find the elapsed time.
Elapsed time is the time between the two given times.
We can also find the starting time, when the finishing time and the duration of activity is given or the finishing time, when the starting time and the duration of the activity is given.
Example 3: Mr. Verma’s office starts at 10 a.m. and closes at 6 p.m. How many hours does the office remain open?
Ans:
Thus, Mr. Verma’s office remains open for 2 hours + 6 hours = 8 hours.
Example 4: A flight took off for Mumbai from New Delhi at 3:10 p.m. The plane landed in Mumbai at 4:55 p.m. What was the duration of the flight?
Ans:
∴ Duration of the flight = 50 minutes + 55 minutes
= 105 minutes = 60 minutes + 45 minutes
= 1 hour + 45 minutes
∵ 1 hour = 60 min
Thus, the duration of the flight was 1 hour 45 minutes.
Example 5: How long is the time from
(a) 1542 hours to 2039 hours?
(b) 4:35 a.m. to 6:20 p.m.?
Ans:
(a) The elapsed time is 2039 hours – 1542 hours.
Thus, the elapsed time is 4 hours 57 minutes.
(b) To find the elapsed time between 4:35 a.m and 6:20 p.m., first convert the time into 24hour time.
Thus, the elapsed time is 13 hours 45 minutes.
Example 6: Find the time using a 24hour clock.
(a) 4 hours 25 minutes after 2040 hours
(b) 8 hours 15 minutes before 2:35 p.m.
Ans:
(a) Add 4 hours 25 minutes to 2040 hours to find the required time.
20 h 40 min + 4 h 25 min
= 24 h 65 min = 24 h + 1 h + 5 min
= 1:05 a.m. (It becomes next day.) 1 h 5 min after 12:00 midnight.
(b) 2:35 p.m. = 1435 hours = 14 h 35 min
∴ Required time = 14 h 35 min – 8 h 15 min
= 6 h 20 min
= 0620 hours or 6:20 a.m.
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