Solved Exercises on Elapsed Time in Grade 3

Master elapsed time: calculating time intervals, solving time word problems, and understanding time duration through these 5 detailed exercises.

Solution: Exercises 1 to 3
1 Simple elapsed time
Exercise 1
If a movie starts at 2:00 PM and ends at 3:30 PM, how long did the movie last?
Definition:

Elapsed time: The amount of time that passes between a start time and an end time

Elapsed time method:
  1. Identify the start time and end time
  2. Subtract the start time from the end time
  3. Calculate the difference in hours and minutes
Start Time
2:00 PM
End Time
3:30 PM
Elapsed Time
1 hour 30 minutes
Step 1: Identify start and end times

Start: 2:00 PM, End: 3:30 PM

Step 2: Calculate the hour difference

From 2:00 PM to 3:00 PM = 1 hour

Step 3: Calculate the minute difference

From 3:00 PM to 3:30 PM = 30 minutes

2:00 PM
Start
3:30 PM
End
Elapsed Time: 1 hour 30 minutes
Final answer:

The movie lasted 1 hour and 30 minutes

Applied rules:

• Subtraction: End time - Start time = Elapsed time

• Same period: Both times in PM, no day change

• Time calculation: Hours and minutes separately

2 Crossing the hour boundary
Exercise 2
If a game starts at 4:45 PM and ends at 5:15 PM, how long did the game last?
Definition:

Crossing hour boundary: When the time interval spans across an hour mark

Start Time
4:45 PM
End Time
5:15 PM
Cross Boundary
Yes
Elapsed Time
30 minutes
Step 1: Identify start and end times

Start: 4:45 PM, End: 5:15 PM

Step 2: Break into segments

From 4:45 PM to 5:00 PM = 15 minutes

Step 3: Continue to end time

From 5:00 PM to 5:15 PM = 15 minutes

Step 4: Add the segments

15 minutes + 15 minutes = 30 minutes

4:45 PM
Start
5:00 PM
Hour Mark
5:15 PM
End
Elapsed Time: 30 minutes
Final answer:

The game lasted 30 minutes

Applied rules:

• Segment approach: Break into parts when crossing hour

• Minute calculation: Count minutes to hour, then from hour

• Addition: Sum the time segments

3 Elapsed time with minutes only
Exercise 3
If a song starts at 1:12 PM and ends at 1:28 PM, how long is the song?
Definition:

Within same hour: When start and end times occur within the same hour

Start Time
1:12 PM
End Time
1:28 PM
Same Hour
Yes
Elapsed Time
16 minutes
Step 1: Identify start and end times

Start: 1:12 PM, End: 1:28 PM

Step 2: Both times in same hour

Both times are in the 1 o'clock hour

Step 3: Subtract minutes only

28 minutes - 12 minutes = 16 minutes

Step 4: Express the result

The song lasted 16 minutes

1:12 PM
Start
1:28 PM
End
Elapsed Time: 16 minutes
Final answer:

The song is 16 minutes long

Applied rules:

• Same hour: Only subtract minutes when hours are identical

• Direct subtraction: End minutes - Start minutes

• Simple calculation: No hour conversion needed

Elapsed Time Fundamentals
\text{Elapsed Time} = \text{End Time} - \text{Start Time}
Elapsed Time Formula
Basic Method
End - Start
Subtract start from end
Same Hour
Minutes Only
Subtract minutes directly
Cross Hour
Break into Segments
Count to hour mark
Key definitions:

Elapsed time: The total amount of time that passes between two events

Start time: When an event begins

End time: When an event finishes

Time interval: The duration between two specific times

4-step elapsed time method:
  1. Identify: Find the start and end times
  2. Compare: Determine if times cross hour boundaries
  3. Calculate: Use appropriate method for the situation
  4. Verify: Check if the answer makes sense
Tip 1: Use a timeline to visualize the time interval.
Tip 2: When crossing hour boundaries, count to the next hour first.
Tip 3: If times are in different periods (AM/PM), consider the full day.
Tip 4: Always check if your answer is reasonable.
Common errors: Forgetting to cross hour boundaries, miscounting minutes, confusing start/end times.
Memory aid: "End minus start gives the time that passed."
Key time facts:

• 60 minutes = 1 hour

• When minutes in start > minutes in end, borrow 1 hour

• Elapsed time is always positive

• Same hour: subtract minutes directly

Solution: Exercises 4 to 5
4 Complex elapsed time
Exercise 4
If a school event starts at 9:30 AM and ends at 11:15 AM, how long was the event?
Definition:

Complex elapsed time: When the time interval crosses multiple hours

Start Time
9:30 AM
End Time
11:15 AM
Cross Hours
9 to 10 to 11
Elapsed Time
1 hour 45 minutes
Step 1: Identify start and end times

Start: 9:30 AM, End: 11:15 AM

Step 2: Break into segments

From 9:30 AM to 10:00 AM = 30 minutes

Step 3: Count full hours

From 10:00 AM to 11:00 AM = 1 hour

Step 4: Count remaining minutes

From 11:00 AM to 11:15 AM = 15 minutes

Step 5: Add all segments

30 minutes + 1 hour + 15 minutes = 1 hour 45 minutes

9:30 AM
Start
10:00 AM
Hour
11:00 AM
Hour
11:15 AM
End
Elapsed Time: 1 hour 45 minutes
Final answer:

The event lasted 1 hour and 45 minutes

Applied rules:

• Segment approach: Break into manageable parts

• Full hours: Count complete hours separately

• Addition: Sum all time segments

5 Real-world elapsed time
Exercise 5
If a train leaves at 2:45 PM and arrives at 4:20 PM, how long was the journey?
Definition:

Real-world elapsed time: Applying elapsed time concepts to practical situations

Departure
2:45 PM
Arrival
4:20 PM
Journey Time
1 hour 35 minutes
Step 1: Identify departure and arrival times

Departure: 2:45 PM, Arrival: 4:20 PM

Step 2: Calculate to next hour

From 2:45 PM to 3:00 PM = 15 minutes

Step 3: Count full hours

From 3:00 PM to 4:00 PM = 1 hour

Step 4: Count remaining minutes

From 4:00 PM to 4:20 PM = 20 minutes

Step 5: Add all time segments

15 minutes + 1 hour + 20 minutes = 1 hour 35 minutes

2:45 PM
Departure
3:00 PM
Hour
4:00 PM
Hour
4:20 PM
Arrival
Journey Time: 1 hour 35 minutes
Final answer:

The train journey lasted 1 hour and 35 minutes

Applied rules:

• Real-world application: Departure and arrival concept

• Segment calculation: Break into logical time chunks

• Total duration: Sum all time segments

Complete Elapsed Time Guide
\text{Elapsed Time} = \text{End Time} - \text{Start Time}
Fundamental Formula
Key definitions:

Time duration: The length of time an event takes place

Interval: The space between two points in time

Chronological calculation: Determining time based on sequence

Advanced calculation techniques:
  1. Timeline approach: Visualize the time span on a number line
  2. Chunk method: Break complex intervals into smaller parts
  3. Borrowing method: When minutes in start > minutes in end
  4. Verification: Check if answer is reasonable
Tip 1: Draw a timeline to visualize the time interval.
Tip 2: Use benchmark times like 30 minutes, 1 hour as reference.
Tip 3: Practice with real-life scenarios like TV shows, meals, activities.
Tip 4: Always verify your answer makes sense in context.
Common errors: Forgetting to cross hour boundaries, misreading times, calculation mistakes.
Success strategies: Regular practice, visual aids, real-world applications.
Essential elapsed time facts:

• Always subtract start time from end time

• When crossing hour boundaries, break into segments

• Elapsed time cannot be negative

• Verify answers by adding elapsed time to start time

Elapsed Time Strategies
Time Interval Approaches
Different methods for calculating elapsed time:
Same hour vs. Cross hour vs. Multiple hours

Strategy Overview:

  • Same hour: Direct minute subtraction
  • Cross hour: Break into segments
  • Multiple hours: Count full hours + remaining minutes
  • Verification: Add elapsed time to start time

Questions & Answers

Question: I get confused when I have to subtract minutes and the start time has more minutes than the end time. For example, 3:15 to 4:05. What do I do?

Answer: This is called "borrowing" in time calculation! Here's how to handle it:

  • When start minutes > end minutes: Borrow 1 hour from the end time
  • Convert borrowed hour: 1 hour = 60 minutes
  • Add to end minutes: 60 + end minutes

For your example (3:15 to 4:05):

  • Since 15 > 5, borrow 1 hour from 4:05
  • 4:05 becomes 3:65 (4 hours - 1 hour = 3 hours, 5 minutes + 60 minutes = 65 minutes)
  • Now subtract: 3:65 - 3:15 = 50 minutes

So the elapsed time is 50 minutes!

Question: How can I check if my elapsed time answer is correct?

Answer: You can verify your answer using the "check method":

  • Add elapsed time to start time: Should equal end time
  • Example: Start 2:00 PM + Elapsed 1 hour 30 minutes = 3:30 PM ✓

You can also:

  • Estimate first: Does your answer seem reasonable?
  • Use a timeline: Draw it out to visualize
  • Count up: Start at the beginning and count forward

Always check your work to catch mistakes!

Question: Do I need to worry about AM and PM when calculating elapsed time?

Answer: For basic elapsed time in grade 3, if both times are in the same period (both AM or both PM), you don't need to worry about it. However:

  • Same period: Just calculate normally (both AM or both PM)
  • Different periods: When going from AM to PM or vice versa, it gets more complex
  • For now: Focus on problems where both times are in the same period

Example: 9:00 AM to 11:30 AM = 2 hours 30 minutes (both AM, so ignore AM/PM)

When you advance to more complex problems, you'll learn about crossing from AM to PM!