Elapsed Time and Empty Number Lines

I cannot think of a better tool to help students understand elapsed time than empty number lines. I was in a Grade 4 class and began by asking them to show me various times on their individual analogue clocks. I have blogged before about the importance of making sure (as teachers and parents) that children are familiar with analogue clocks so I was happy to see how adept all these students were with showing me a variety of times. As it was 8:30, I set them a challenge:

What will you be doing in two-and-a-half hours?

As I moved around the class listening to and watching the students thinking, I noticed that some were using the clocks to help them find the new time whilst others seemed to be using their fingers to keep track of their thinking. There were two common answers: 11:00 and 10:00.  

I then told them how I 'see' this problem using an empty number line.

First, I mark the start time on my line:

Then I move forward two hours from this:

And finally add the extra 30 minutes:

One student then said that this was like how they have learned to add and subtract numbers: a good sign!

When we agreed that the time would be 11:00, they were able to tell me that they would just be beginning lunch.

I then gave them this challenge:

You are going to see a film that starts at 3:15 p.m. You know that it lasts three-and-a-half hours. What time will it finish?

As I moved around the room, I could see students making great progress by using the number line:

Some students used an analogue clock to help their thinking:

All were able to give me the correct time that the film finishes.
My next challenge was this:
Your train leaves Oshawa at 9:15 a.m. and takes 6 hours and 40 minutes to get to its destination. What time will it arrive. Again, the students used the empty number line effectively:

What I like about number lines, is that students use numbers that are friendly them and that these eventually will lead to more efficient strategies such that students (in my experience) begin to 'see' the number line in their head.
I ended the lesson by giving this challenge:

It is now 9:25 a.m. If school ends at 2:50 p.m., how many minutes are there before you go home

Unfortunately, I forgot to take pictures of their solutions, but they were able to get the correct solution despite not being shown how to do problems like this: they just used the empty number line, marked on the start time and end time then figured out the elapsed time in ways like this: