HSPT: Time: add minutes to a clock time
Learn to find an end time or a length of time on a 12-hour clock, carrying 60 minutes into an hour and keeping a.m. and p.m. straight.
How do you find when something ends if you know its start time and how many minutes it lasts?
Short answer
Change the minutes into hours and minutes, add them to the start time, and carry every 60 minutes into an hour.
Know first: 60 minutes in an hour, a.m. and p.m.
Step 1 of 6
Clocks carry at 60, not 100
Adding time is like adding numbers, with one difference: 60 minutes make an hour. When the minutes reach 60 or more, subtract 60 and add 1 to the hours.
First change a long stretch of minutes into hours and minutes: 85 minutes is 1 hour 25 minutes. Then add hours to hours and minutes to minutes, and carry if needed.
| Minutes | Hours and minutes |
|---|---|
| 75 | 1 h 15 min |
| 85 | 1 h 25 min |
| 110 | 1 h 50 min |
| 140 | 2 h 20 min |
Step 2 of 6
Add, then carry the extra hour
Worked example: An appointment that runs long
An appointment starts at 9:48 a.m. and lasts 85 minutes. When does it end?
- A11:13 a.m.
- B10:33 a.m.
- C10:13 a.m.
- D12:13 p.m.
- 1
Convert
85 minutes = 1 hour 25 minutes.
- 2
Add
9:48 + 1:25 gives 10 hours and 73 minutes.
- 3
Carry
73 minutes = 1 hour 13 minutes, so the time is 11:13 a.m.
- 4
See why the other choices are there
10:33 adds 85 to 948 as if an hour had 100 minutes. 10:13 drops the 60 minutes without adding the hour. 12:13 adds an hour but keeps all 85 minutes too.
Answer
11:13 a.m.
Find the wrong step
A student finds when a 50-minute class ends if it starts at 11:40 a.m.
- 1
Adds the minutes: 40 + 50 = 90.
- 2
Carries 60 minutes: 90 minutes is 1 hour 30 minutes, so the time is 12:30.
- 3
Writes the end time as 12:30 a.m.
What went wrong
Passing 12:00 in the late morning moves the clock to p.m. 12:30 a.m. would be just after midnight.
The fix
The class ends at 12:30 p.m.
Step 3 of 6
Find how long something lasts
To find the time between two clock readings, count up in friendly jumps: first to the next full hour, then by whole hours, then the leftover minutes.
From 7:50 to 9:05: 7:50 to 8:00 is 10 minutes, 8:00 to 9:00 is 60 minutes, and 9:00 to 9:05 is 5 minutes. That is 10 + 60 + 5 = 75 minutes.
Step 4 of 6
Carry at 60 and watch a.m. and p.m.
Check yourself · Question 1
A movie starts at 6:15 p.m. and runs 110 minutes. When does it end?
Answer: C
110 minutes = 1 hour 50 minutes. 6:15 + 1:50 gives 7 hours 65 minutes. Carry 60 minutes: 8:05 p.m.
Trap answer: 7:25 p.m.
Hundreds instead of sixties
- Why it looks right
- Splitting 110 into 1 and 10 looks just like place value with ordinary numbers.
- Why it's wrong
- An hour has 60 minutes, so 110 minutes is 1 hour and 50 minutes.
- The right answer
- 8:05 p.m.
Check yourself · Question 2
Soccer practice runs from 3:40 p.m. to 5:25 p.m. How many minutes long is it?
Answer: C
Count up: 3:40 to 4:00 is 20 minutes, 4:00 to 5:00 is 60 minutes, and 5:00 to 5:25 is 25 minutes. 20 + 60 + 25 = 105 minutes.
Common mistake
"I add the minutes to the clock time like ordinary numbers."
- Why it's tempting
- Clock times look like numbers, so ordinary addition feels natural.
- Do this instead
- Change minutes into hours and minutes first, and carry every 60 minutes into an hour.
Step 5 of 6
What to remember
Remember
- Change long stretches of minutes into hours and minutes before adding.
- Carry 60 minutes into 1 hour, and switch a.m. and p.m. when you pass 12:00.
- To find how long something lasts, count up to the next hour, then by hours, then the extra minutes.
Step 6 of 6
Practice on a real question
Use what you just learned on this question, then check the explanation.
In your own words
How many hours and minutes was the practice question's length of time, and where did you carry 60 minutes?