So far, you've learned about the different units of time, converting between units of time, measuring elapsed time, and time zones.
We can use this knowledge to solve real world problems.
Imagine a parent is trying to finish a number of tasks before picking up their child from school.
She wants to clean the house for 30 minutes...
do laundry for 45 minutes....
go to the supermarket for 1 hour and 15 minutes...
And drive for 10 minutes to her child's school.
If she starts doing all these at 1:00 p.m., will she make it to her kid's school at 3:00 p.m.?
It looks like we need to add and subtract different units of time to figure out the answer.
The most common units of time are seconds, minutes and hours.
Second is the smallest unit.
Next is the minute.
The largest unit is the hour.
1 minute = 60 seconds
1 hour = 60 minutes
So, if we have 120 minutes, how many hours is that?
When we convert from a smaller unit, like seconds, to a larger unit, like minutes, we divide.
Since minute is smaller than hour, we'll divide to get the answer.
120 ÷ 60 = 2
Now, we know that 120 minutes = 2 hours.
We can add time units using column form.
To add time measurements, add the like, or similar, units first.
Add seconds to seconds, minutes to minutes, and hours to hours.
Tip: Add starting with the smallest unit.
In our problem, "minute" is smaller than hour.
Then, simplify the answer by converting the units.
Tip: If the answer for "minute" is 60 or more, it means that you need to regroup 60 minutes into an hour.
100 minutes is equal to 1 hour and 40 minutes.
1 hour + 1 hour and 40 minutes = 2 hours and 40 minutes
The total time spent on doing chores and errands is 2 hours and 40 minutes. ✅
Great work.
She starts at 1:00 p.m.
It will take her 2 hours and 40 minutes to finish everything.
Let's use the idea of elapsed time to answer the question.
2 hours and 40 minutes after 1:00 p.m. is 3:40 p.m.
This means that this mother won't be able to make it to her children's school at 3:00 p.m.
It looks like she has to take out a chore from her list.
Let's learn to subtract time units with an example:
5 min 20 sec minus 3 min 45 sec = ?
We can use column form again.
Tip: You can shorten the names of the time units.
second - sec
minute - min
hour - hr
Subtract starting from the smallest unit.
We can't subtract 45 seconds from 20 seconds.
45 is more than 20!
So we borrow from the minute unit.
We borrowed 1 minute from 5 minutes.
1 minute is equal to 60 seconds. We added it to 20 seconds.
Now, our minuend is 4 minutes and 80 seconds.
Let's subtract.
Now, we've got our answer.
5 min 20 sec minus 3 min 45 sec = 1 minute and 35 seconds
Great job.
Let's try another example:
3 hr and 5 sec minus 4 min = ?
Let's write this problem in column, or vertical, form.
There are no like units in this problem.
So we have to convert one of these time units.
Which should we convert?
Let's convert the 3 hours in the minuend into minutes.
Let's set aside the 5 seconds for now.
3 hours = _______ minutes
We know that there are 60 minutes in 1 hour.
To convert hours into minutes, we multiply by 60.
3 hours × 60 = 180 minutes
So, we can rewrite our minuend as 180 minutes and 5 seconds.
Let's subtract:
Our difference is 176 minutes and 5 seconds.
Set aside 5 seconds. Focus on 176 minutes.
How can we simplify 176 minutes?
Let's divide 176 by 60.
176 minutes is equal to 2 hours and 56 minutes.
Let's add that to 5 seconds.
2 hours 56 minutes + 5 seconds
Our final answer is...
3 hours and 5 seconds minus 4 minutes = 2 hours, 56 minutes and 5 seconds
Great work. 👏
Now, you're ready for the practice.