Problem
Elapsed time between two clock times
A workout began at 1:20 and ended at 2:40. Find how long it lasted, in hours and minutes.
Measurement & data
Your answer
How to solve
Strategy Draw a Diagram — Use the time band as a number line: jump from 1:20 to 2:20 (a whole hour), then from 2:20 to 2:40 (the remaining minutes). Adding the two jumps gives the total elapsed time.
1STEP 1
Jump one whole hour
From 1:20 to 2:20 is exactly 1 hour.
1{:}20 → 2{:}20 = 1 hour
Moving the hour by one and keeping the same minutes is a clean 1-hour hop on the time line.
3.MD.A.1Draw A Diagram2STEP 2
Count the leftover minutes
From 2:20 to 2:40 the minute hand moves from 20 to 40, which is 20 minutes.
40 - 20 = 20 minutes
Same hour, so just subtract the minute numbers to get the extra time.
2.MD.C.7Identify Subproblems3STEP 3
Add the two parts
Total elapsed time is 1 hour plus 20 minutes.
1 hour + 20 minutes = 1 hour 20 minutes
Combining the whole-hour jump and the minute jump gives the full duration.
3.MD.A.1Identify SubproblemsThe whole elapsed time from 1:20 to 2:40 equals the 1-hour jump plus the 20-minute jump added together.
Why?
On the time line the two jumps sit end to end — the first ends at 2:20, exactly where the second begins — so together they reach from 1:20 all the way to 2:40 with nothing skipped and nothing counted twice.
Why?
When pieces cover a whole span leaving no gap and no overlap, their lengths add back to the length of the whole span.
🧱Whole is the sum of its partsBreak a thing into pieces with no gaps or overlaps and the pieces add back to the whole.
Answer
1 hour 20 minutes
Check by adding the duration back to the start: 1:20 + 1 hour = 2:20, then + 20 minutes = 2:40, which matches the end time. So 1 hour 20 minutes is correct.
Takeaway
Hop one whole hour, then count the extra minutes: easy Grade 3 elapsed-time counting!
- Jump one whole hour
- Count the leftover minutes
- Add the two parts
Where next?
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