This year when I was teaching elapsed time, I taught it with the backwards N like I always do, and with timelines.  So many of my kids got it, and rocked it, and moved on through it.  For some of my kids, though, they just couldn’t get it.  They couldn’t remember what numbers went where, and it just didn’t click with them.  So, I had to try something new.  This method worked for them pretty much immediately, so I thought I’d share.  I call it the T-Method because the first thing students do is start out with a lowercase t on their paper.  We also used the same method when we were doing perimeter and needed to add lengths together, or subtract lengths in story problems.

I will say that some of them sound a little bit complicated once you start doing lots of conversions.  However, once students understand it with easier problems, it’s really not complicated for them at all.

For problems where the start time and the amount of time that has passed are known the following steps are used.

First, the start time is included and then the elapsed time is included.  Each side is added independently of each other.  Then, students identify if they need to regroup into a new hour.  I use the term regroup because it’s what we use with addition and subtraction and help related it for them.  The final time is then left.

Because we use the language START, CHANGE, END when we are working on addition and subtraction at the beginning of the year, it transfers right over to elapsed time.  Subtraction is done when the end time is given, but either the start time or the time that has passed are not.

This is also an example of when students are unable to complete each side independently.  Students immediately identify that they cannot do 20-40, and that they need to regroup to continue.  Once they have a new starting time, they can subtract each side and have the missing time.

This is when things get a little tricky.  Upon first looking at this you would think I’d lost my mind.  But here’s an example story problem.

Sarah’s family was driving across the country on a road trip.  They reached Florida at 2:20 a.m., and immediately went to bed.  They drove for 12 hours and 40 minutes.  What time must they have started their drive?

Because the start is unknown, it’s a subtraction problem.  First, students identify that since they traveled after midnight, they have to convert into daytime.  They add 12 hours to the start time.


Once that’s completed, they have a new start time.  Now, they are unable to do the minutes, so they again regroup.  Now, again, this seems like a lot of steps and quite complicated. Because it’s 12 hours I also talk to my students how that 12 just basically cancels itself out.  But if 11 hours were used in this example, the steps would be used.  Once students have mastered the steps from above, it really is simple for them to add the one additional step.

We also transferred the same steps to adding and subtracting lengths and then converting them.

Again, each side is done independently of the other until the end when it’s analyzed to see if it needs to be regrouped.

While this takes me back to my days in Geometry doing proofs over and over again, that little line in the middle really helps my kids not just carry and borrow between.

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