December 2, 2017

Elapsed Time Using an Open Number Line

Elapsed Time Using an Open Number Line

Determining elapsed time can be a huge issue for many students for several reasons:

It often involves the conversion of units.  For example, hours to minutes, minutes to seconds, etc.

Students are often confused as to whether they should more forward (from the earlier time to the later time) or backwards as they start to determine elapsed time.

When given the elapsed time and either a starting OR ending time, and asked to determine the missing piece of information (starting or ending time), students often want to simply add (or subtract) the given elapsed time as a total.  For example, if a question involved an activity that started at 9:50 and lasted 1 hour and 15 minutes, students often simply add 950 + 115= 1065, and record that the final time was 10:65.

Open Number Line to the Rescue!

Once I feel that my Grade 5/6 students have a good understanding of the unit conversions (ie. 60 seconds= 1 minute, 60 minutes = 1 hour, etc.) and are ready to work with elapsed time situations, we use an open number line.  Because my students do "number talks" every day to strengthen their mental math skills, they are already familiar with closed and open number lines.  

Sample question:  It is now 9:45 and our recess bill rings at 11:10.  Determine how much time will pass before the bell rings.

Students begin by drawing a straight line across their desks. (Yes, I have them write directly on their desks with whiteboard markers.  It saves a TON of paper, and it's a snap to clean up with a dampened old sock, piece of felt, or other material.)

They then record the beginning time (9:45) at the start of their line, and the finishing time (11:10) at the end of their line.  

Then students work in "chunks" of time, usually working from the left to right, to determine the elapsed time.  There are an infinite number of ways to chunk, depending upon the particular comfort level of your students. 

Grade 5 Elapsed Time Using an Open Number Line

This Grade 5 student is most comfortable starting with smaller units and working up to the nearest hour.  She jumps by 5 minutes to 9:50, and then by 10 minutes to 10:00.  From there she was able to continue on to add an hour from 10:00 to 11:00, and then add 10 minutes on to reach the ending time of 11:10.  She then added the minutes together (5+10+10=25) and attached them to the hour, reaching the correct time of 1 hour and 25 minutes.

Grade 6 Elapsed Time Using an Open Number Line
Meanwhile, this Grade 6 student approached the situation a bit differently, first adding a full hour from 9:45 to 10:45, and then chunking by 15 minutes to reach 11:00 and then another 10 minutes to reach the ending time.  This method makes it very easy for this student to determine the elapsed time (1 hour plus 15 minutes plus 10 minutes = 1 hour & 25 minutes.

The joy of this open number line strategy is that students can use it working either forwards OR backwards, depending on each student's comfort level.  This was particularly important in my split-grade classroom last year, which contained a very wide range of ability levels.  Everyone met with success, with each student deciding on how best to "chunk" measurement units and whether to work forwards or backwards.  

While this is not the only strategy for determining elapsed time, it is the one that has met with the greatest success in my classes.  What have YOU found to be helpful in approaching this challenging concept?


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