Problem
Elapsed time between two clock times
A workout started at 1:20 and ended at 2:40. How long did the workout take, in hours and minutes?
Measurement & data
Your answer
Watch out Don't assume only 20 minutes passed just because the minute hand moved from 20 to 40 — the hour hand also crossed from 1 to 2, so you need to add that whole hour first.
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) first, 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, exactly 1 hour passes.
1:20 → 2:20 = 1 hour
Raising the hour by one while keeping the same minute mark is exactly a one-hour hop on the time line.
Draw A Diagram3.MD.A.1Time2STEP 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
Within the same hour, subtracting the minute numbers directly gives the extra time — since the hour hasn't changed, comparing minutes alone is enough.
Identify Subproblems2.MD.C.7Time3STEP 3
Add both parts for the total time
Adding 1 hour and 20 minutes gives a total of 1 hour 20 minutes.
1 hour + 20 minutes = 1 hour 20 minutes
The two jumps sit end to end on the time line — the first ends at 2:20, exactly where the second begins — so adding them covers the whole elapsed time with nothing skipped or double-counted.
Identify Subproblems3.MD.A.1TimeThe 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
1 hour + 20 minutes = 1 hour 20 minutes
Check — adding 1 hour to 1:20 gives 2:20, then adding 20 minutes gives 2:40, matching the end time. So 1 hour 20 minutes is correct.
Takeaway
The gap between two clock times is a duration. Hop the whole hour first, then add the leftover minutes to get the total elapsed time.
- 1:20 → 2:20 = 1 hour
- 2:20 → 2:40 = 20 minutes
- Add both parts → 1 hour 20 minutes
Where next?
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