Concept 1 of 7
Elapsed Time: The Bridge Method
Today's big idea
Hop to the next whole hour, across the whole hours, then to the end time, and add the hops.
Hopping never needs a borrow; subtracting clock times almost always does.
How it works
👀 What's going on?
Clock times refuse to behave like ordinary numbers. There are 60 minutes in an hour, not 100, so subtracting them goes wrong fast. Hopping forward is far safer.
💡 Here's the trick
Look at the bridge above. You do not subtract the two times at all. You hop up to the next whole hour, then across the whole hours, then on to the end time. Adding the three hops gives the answer.
🪄 How to do it
- 1.Hop from the start time to the next whole hour, as the bridge above shows.
- 2.Hop across the whole hours to the hour before the end time.
- 3.Hop on to the end time, then add all three hops.
✍️ Try it
A film runs from 10:35 AM to 1:20 PM. Then: a bus leaves at 6:50 AM and reaches town at 9:05 AM. How long is each one?
The film hops 25 minutes to 11:00 AM, then 2 hours to 1:00 PM, then 20 minutes, so it runs 2 hours 45 minutes. The bus hops 10 minutes, then 2 hours, then 5 minutes, giving 2 hours 15 minutes.
⚠️ Watch out
You might be thinking you can just subtract the two clock readings. Try that on 10:35 AM and 1:20 PM and you get nonsense. Once a time crosses noon or midnight, only hopping keeps you right.
🌟 Why this matters
You use this to work out how long a train ride, a nap or a birthday party lasted. Cooks use it for oven timings and coaches use it for match lengths.
Exam tip: Hop to the next whole hour first, and you never need to borrow 60.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“How long is it from 2:40 PM to 5:15 PM?”
- 1
Bridge to the next whole hour
2:40 → 3:00 = 20 min. 🌉
- 2
Cross to the target hour
3:00 → 5:00 = 2 h.
- 3
Land at the end time
5:00 → 5:15 = 15 min.
- 4
Add the three ✓
20 min + 2 h + 15 min = 2 h 35 min
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Bridge in 3 jumps: up to the next whole hour → across the hours → onto the end. Then add.
ELAPSED · ROUND 1 OF 3
9:50 AM → 11:20 AM
Quick check
✅ VERIFICATION
A film starts at 7:20 PM and runs for 2 hours 50 minutes. When does it end?
⚡ Finish the activity above first to answer.
Keep going!
Up next: 12-Hour and 24-Hour
PM hours? Add 12. After-noon → 13:00 onwards.