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Addition and Subtraction of Time II

Here are some more examples of addition and subtraction of time periods:

 

Example 1: What fraction of a day is 3 hours?

1 day = 24 hours. So

{3~hours}/{24~hours}  =  3/24  =  1/8

 

Example 2: What fraction of 12 hours is 8 hours?

{8~hours}/{12~hours}  =  2/3

 

Example 3: What fraction of 2 days is 8 hours?

2 days = 2 x 24 = 48 hours.

Therefore {8~hours}/{48~hours}  =  1/6

 

Example 4: What fraction of 11/2 hours is 40 minutes?

11/2 hours  =  90 minutes

Therefore the fraction is 40/90  =  4/9

 

Example 5: A game started at 7:52 pm, and finished at 8:32 pm. How long did the game last?

8 hours 32 minutes
7 hours 52 minutes

 

Since we can’t subtract 52 from 32, the question is changed to get the answer –

7 hours 92 minutes
7 hours 52 minutes

40 minutes

The game was played for 40 minutes.

 

Example 6: A Thai Airlines flight left Bangkok at 8:35 am, and landed at Singapore at 12:52 pm. After a refuelling stop of 30 minutes, the plane took off for Dubai, and landed there at 9:42 pm. Calculate the total journey time of the aircraft. What is its total flying time?

To calculate the total flying time of the aircraft, we need to calculate the flying times of the two legs of the journey, viz. Bangkok-Singapore and Singapore-Dubai, and add them up.

First the flying time between Bangkok and Singapore =

12 hours 52 minutes
8 hours 35 minutes

4 hours 17 minutes

 

The plane stopped at Singapore for 30 minutes to refuel. So we add 30 minutes to the arrival time to get the departure time for its second leg to Dubai.

12 hours 52 minutes +
30 minutes

13 hours 22 minutes

 

So the departure time of the plane in Singapore is 1322 hours, or 1:22 pm. The flight arrived at Dubai at 9:42 pm, so the flying time for the second leg is –

9 hours 42 minutes
1 hour 22 minutes

8 hours 20 minutes

So the total journey time for the aircraft is –

4 hours 17 minutes +
8 hours 20 minutes

12 hours 37 minutes

 

To get the total flying time, we subtract the total journey time by 30 minutes (for refuelling). So the total flying time is 12 hours 7 minutes.