Elapsed time is the amount of time that passes between a start time and an end time. Calculating elapsed time is tricky because time uses a base-60 system (60 minutes in an hour, 60 seconds in a minute) rather than base-10. Students learn to find elapsed time, end times, and start times in problems like "A movie starts at 2:45 PM and is 1 hour 35 minutes long. When does it end?" This is a practical life skill and also reinforces flexible reasoning with non-decimal number systems.
Use open number lines: jump from the start time to the end time in convenient chunks (jump to the next hour, then add remaining minutes). Analog clocks help students see the movement of time visually. Practice with real schedules (bus timetables, class schedules, cooking times). Avoid teaching a single algorithm -- flexible "jump" strategies build better number sense.
Elapsed time is the amount of time that passes between two moments — the answer to "how long did that take?" or "when does it finish?" You already know how to read a clock to the minute, which means you can identify start and end times precisely. What makes elapsed time tricky is that time uses a base-60 system: 60 minutes make an hour, not 100. That single fact breaks every instinct you have built around our usual base-10 number system.
The most powerful tool for elapsed time is an open number line. Draw a line, mark the start time on the left, and jump toward the right in convenient chunks — first to the next whole hour, then in larger hour-sized hops, then in smaller minute-sized adjustments at the end. For example, to find how long it takes from 10:45 AM to 1:20 PM: jump 15 minutes to get to 11:00 (a clean hour boundary), then jump 2 hours to get to 1:00 PM, then jump 20 minutes to 1:20 PM. Total: 15 min + 2 hr + 20 min = 2 hours 35 minutes. Notice how hitting the "clean hour" first simplifies everything — it is the same strategy as making change by counting up to a round dollar. The number line makes the jumps visible and prevents the base-60 trap.
The base-60 trap looks like this: a movie starts at 2:45 and lasts 1 hour 35 minutes, so you try to compute 2:45 + 1:35. If you treat the minutes like a normal base-10 addition, you get 3:80 — but 3:80 is not a real time. The correct answer is 4:20, because when the minutes exceed 60, you carry one hour and keep the remainder: 45 + 35 = 80 minutes = 1 hour and 20 minutes, then 2 + 1 + 1 = 4 hours. The jump strategy sidesteps this problem by keeping you anchored to real clock positions throughout, rather than doing raw arithmetic that might violate the 60-minute boundary. Once you are fluent with forward jumps, you can run the same process in reverse — jumping backwards from an end time to find when something started.
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