Elapsed time

In this lesson, we will learn:

• How to find elapsed time between a start time and an end time with 3 methods: counting hours and minutes separately, counting minutes by 5s, and using a number line
• How to find the time after elapsed amount and the time before an elapsed amount by doing time addition and time subtraction

Notes:

• Elapsed time means how much time has passed. There will be three types of questions:
• Figuring out how much elapsed time was between 2 given times
• Figuring out what time it will be from a given time after passing an elapsed time
• Figuring out what time it was for an elapsed time before a given time


• Three methods to find out the elapsed time between 2 given times are: (1) Counting the hours and minutes separately, (2) Counting by 5s, (3) Showing elapsed time on a number line
• (1) Counting the hours and minutes separately only works if one time has both greater hours and minutes than the other
• Ex. from 6:20AM to 7:55AM: hours (6 to 7) is 1h between; minutes (20 to 55) is 35min between $\,$→$\,$ the elapsed time is 1h 35min
• (2) Counting by 5s works best when the minutes are all multiples of 5
• Ex. from 4:45PM to 5:15PM $\,$→$\,$ starting from 0 for the start time, continue counting by 5s and adding to the time until the end time
• $\large \frac{4 \, : \, 45}{0}$ $\,$→$\,$ $\large \frac{4 \, : \, 50}{5}$ $\,$→$\,$ $\large \frac{4 \, : \, 55}{10}$ $\,$→$\,$ $\large \frac{5 \, : \, 00}{15}$ $\,$→$\,$ $\large \frac{5 \, : \, 05}{20}$$\,$→$\,$ $\large \frac{5 \, : \, 10}{25}$ $\,$→$\,$ = 30 min
• (3) Showing the elapsed time on a number line works well for longer durations of elapsed time (many hours)
• It does not have to be scaled (size of divisions $\neq$ amount of time)
• From the start time, count by 5s to the next exact hour. Then show the next consecutive hours until the hour for the end time. Finally, add on the minutes for the end hour.
• Ex. from 10:30AM to 6:20PM $\,$→$\,$ the elapsed time is 7h 50min

• When figuring out what time it will be from a given time after passing an elapsed time, we will be adding elapsed time (in 24-hour time on a grid with colons, like adding decimals)
• For regrouping, 60 minutes becomes 1 hour
• Ex. 1:75 can be regrouped to become 2:15
• When figuring out what time it was for an elapsed time before a given time, we will be subtracting elapsed time (in 24-hour time on a grid with colons, like subtracting decimals)
• For exchanging, 1 hour becomes 60 minutes
• Ex. 5:32 can be exchanged to become 4:92