3B · Chapter 14 · Lesson 3
Calculating Time — Part 2
Find start or finish times from a known duration.
Today's mathematical question
A lesson begins at 12:50 p.m. and lasts 50 minutes. When does it finish?
Learning target
Find start or finish times from a known duration.
Before starting: Prior lesson: Calculating Time — Part 1. Try the warm-up independently.
Prerequisite · 3 questions
Check earlier skills first
These are earlier ideas you need for today; revisit them if they are difficult.
- P1
How many minutes pass from 10:00 a.m. to 10:50 a.m.?
Reveal explanation
Answer: 50 min
Count from 10:00 a.m. to the next whole hour, then onward to 10:50 a.m.. The intervals add to 50 minutes.
Analog clock showing 10:00 a.m.. - P2
How many minutes pass from 12:45 p.m. to 1:50 p.m.?
Reveal explanation
Answer: 65 min
Count from 12:45 p.m. to the next whole hour, then onward to 1:50 p.m.. The intervals add to 65 minutes.
Analog clock showing 12:45 p.m.. - P3
How many minutes pass from 12:40 p.m. to 1:55 p.m.?
Reveal explanation
Answer: 75 min
Count from 12:40 p.m. to the next whole hour, then onward to 1:55 p.m.. The intervals add to 75 minutes.
Analog clock showing 12:40 p.m..
Learn and understand
If the starting time and duration are known, calculate the finishing time by counting forward. First add enough minutes to reach the next full hour, then add the remaining duration. Always track whether the hour has moved from a.m. into p.m. For a 35-minute lesson starting at 2:50 p.m., count 10 minutes to 3:00 and then 25 minutes to 3:25 p.m. Label the ending time correctly.
Represent it three ways
- Concrete: Build equal pieces, measures, counters or shapes with real objects when useful.
- Pictorial: Plot the start on an open timeline. Break the duration into a jump to the hour and the time remaining, then label the finish.
- Abstract: Write the matching number sentence, calculation, comparison or mathematical description. Explain each number's unit.
Important vocabulary
- start time
- when event begins
- finish time
- when event ends
- duration
- time spent
Fully worked examples and checks
Try each new understanding check before revealing the explanation.
Worked example 1
Explore, calculate, verify
A lesson begins at 12:50 p.m. and lasts 50 minutes. When does it finish?
- Identify the whole, the units, and what the question asks.
- Add 50 minutes to the start time, crossing each next hour carefully. 12:50 p.m. plus 50 minutes is 1:40 p.m..
- Check that the answer uses the right units and makes sense.
Answer: 1:40 p.m.
Check your understanding 1
A lesson begins at 12:40 p.m. and lasts 110 minutes. When does it finish?
Reveal answer and explanation
2:30 p.m. — Add 110 minutes to the start time, crossing each next hour carefully. 12:40 p.m. plus 110 minutes is 2:30 p.m..
Worked example 2
Explore, calculate, verify
A lesson begins at 10:30 a.m. and lasts 35 minutes. When does it finish?
- Identify the whole, the units, and what the question asks.
- Add 35 minutes to the start time, crossing each next hour carefully. 10:30 a.m. plus 35 minutes is 11:05 a.m..
- Check that the answer uses the right units and makes sense.
Answer: 11:05 a.m.
Check your understanding 2
A lesson begins at 6:20 a.m. and lasts 135 minutes. When does it finish?
Reveal answer and explanation
8:35 a.m. — Add 135 minutes to the start time, crossing each next hour carefully. 6:20 a.m. plus 135 minutes is 8:35 a.m..
Worked example 3
Explore, calculate, verify
A lesson begins at 6:35 a.m. and lasts 25 minutes. When does it finish?
- Identify the whole, the units, and what the question asks.
- Add 25 minutes to the start time, crossing each next hour carefully. 6:35 a.m. plus 25 minutes is 7:00 a.m..
- Check that the answer uses the right units and makes sense.
Answer: 7:00 a.m.
Check your understanding 3
A lesson begins at 3:05 p.m. and lasts 35 minutes. When does it finish?
Reveal answer and explanation
3:40 p.m. — Add 35 minutes to the start time, crossing each next hour carefully. 3:05 p.m. plus 35 minutes is 3:40 p.m..
Guided · 5 questions
Guided practice
Keep the model and earlier examples visible while practising.
- G1
A lesson begins at 1:45 p.m. and lasts 50 minutes. When does it finish?
Reveal explanation
Answer: 2:35 p.m.
Add 50 minutes to the start time, crossing each next hour carefully. 1:45 p.m. plus 50 minutes is 2:35 p.m..
Analog clock showing 1:45 p.m.. - G2
A lesson begins at 11:40 a.m. and lasts 50 minutes. When does it finish?
Reveal explanation
Answer: 12:30 p.m.
Add 50 minutes to the start time, crossing each next hour carefully. 11:40 a.m. plus 50 minutes is 12:30 p.m..
Analog clock showing 11:40 a.m.. - G3
A lesson begins at 12:00 p.m. and lasts 110 minutes. When does it finish?
Reveal explanation
Answer: 1:50 p.m.
Add 110 minutes to the start time, crossing each next hour carefully. 12:00 p.m. plus 110 minutes is 1:50 p.m..
Analog clock showing 12:00 p.m.. - G4
A lesson begins at 10:20 a.m. and lasts 90 minutes. When does it finish?
Reveal explanation
Answer: 11:50 a.m.
Add 90 minutes to the start time, crossing each next hour carefully. 10:20 a.m. plus 90 minutes is 11:50 a.m..
Analog clock showing 10:20 a.m.. - G5
A lesson begins at 10:10 a.m. and lasts 135 minutes. When does it finish?
Reveal explanation
Answer: 12:25 p.m.
Add 135 minutes to the start time, crossing each next hour carefully. 10:10 a.m. plus 135 minutes is 12:25 p.m..
Analog clock showing 10:10 a.m..
Independent · 8 questions
Independent practice
Work these without hints and explain the method used. Write answers on paper.
- I1
A lesson begins at 11:30 a.m. and lasts 75 minutes. When does it finish?
Reveal explanation
Answer: 12:45 p.m.
Add 75 minutes to the start time, crossing each next hour carefully. 11:30 a.m. plus 75 minutes is 12:45 p.m..
Analog clock showing 11:30 a.m.. - I2
A lesson begins at 6:15 a.m. and lasts 35 minutes. When does it finish?
Reveal explanation
Answer: 6:50 a.m.
Add 35 minutes to the start time, crossing each next hour carefully. 6:15 a.m. plus 35 minutes is 6:50 a.m..
Analog clock showing 6:15 a.m.. - I3
A lesson begins at 2:40 p.m. and lasts 75 minutes. When does it finish?
Reveal explanation
Answer: 3:55 p.m.
Add 75 minutes to the start time, crossing each next hour carefully. 2:40 p.m. plus 75 minutes is 3:55 p.m..
Analog clock showing 2:40 p.m.. - I4
A lesson begins at 4:05 p.m. and lasts 35 minutes. When does it finish?
Reveal explanation
Answer: 4:40 p.m.
Add 35 minutes to the start time, crossing each next hour carefully. 4:05 p.m. plus 35 minutes is 4:40 p.m..
Analog clock showing 4:05 p.m.. - I5
A lesson begins at 7:35 a.m. and lasts 65 minutes. When does it finish?
Reveal explanation
Answer: 8:40 a.m.
Add 65 minutes to the start time, crossing each next hour carefully. 7:35 a.m. plus 65 minutes is 8:40 a.m..
Analog clock showing 7:35 a.m.. - I6
A lesson begins at 10:50 a.m. and lasts 110 minutes. When does it finish?
Reveal explanation
Answer: 12:40 p.m.
Add 110 minutes to the start time, crossing each next hour carefully. 10:50 a.m. plus 110 minutes is 12:40 p.m..
Analog clock showing 10:50 a.m.. - I7
A lesson begins at 12:45 p.m. and lasts 35 minutes. When does it finish?
Reveal explanation
Answer: 1:20 p.m.
Add 35 minutes to the start time, crossing each next hour carefully. 12:45 p.m. plus 35 minutes is 1:20 p.m..
Analog clock showing 12:45 p.m.. - I8
A lesson begins at 12:00 p.m. and lasts 65 minutes. When does it finish?
Reveal explanation
Answer: 1:05 p.m.
Add 65 minutes to the start time, crossing each next hour carefully. 12:00 p.m. plus 65 minutes is 1:05 p.m..
Analog clock showing 12:00 p.m..
Challenge · 2 questions
Challenge and error analysis
Go beyond arithmetic: justify a claim, correct reasoning, or connect two representations.
- C1
Explain how 11:50 a.m. plus 40 minutes crosses noon. At what time does it finish?
Reveal explanation
Answer: 12:30 p.m.
10 minutes reach noon; then 30 more minutes reach 12:30 p.m.
- C2
A trip begins at 11:45 p.m. and lasts 45 minutes. On which day does it finish and at what time?
Reveal explanation
Answer: Next day at 12:30 a.m.
15 minutes reach midnight; 30 minutes more reach 12:30 a.m. on the next day.
Exit · 3 questions
Exit check
Solve independently, then return on another day to check retention.
- E1
A lesson begins at 11:35 a.m. and lasts 135 minutes. When does it finish?
Reveal explanation
Answer: 1:50 p.m.
Add 135 minutes to the start time, crossing each next hour carefully. 11:35 a.m. plus 135 minutes is 1:50 p.m..
Analog clock showing 11:35 a.m.. - E2
A lesson begins at 1:45 p.m. and lasts 110 minutes. When does it finish?
Reveal explanation
Answer: 3:35 p.m.
Add 110 minutes to the start time, crossing each next hour carefully. 1:45 p.m. plus 110 minutes is 3:35 p.m..
Analog clock showing 1:45 p.m.. - E3
A lesson begins at 2:30 p.m. and lasts 75 minutes. When does it finish?
Reveal explanation
Answer: 3:45 p.m.
Add 75 minutes to the start time, crossing each next hour carefully. 2:30 p.m. plus 75 minutes is 3:45 p.m..
Analog clock showing 2:30 p.m..