Elapsed Time...a race to the finish!

We have been working on elapsed time in math class recently and I have discovered that my students are pretty good at converting between digital and analog time but are struggling with how to solve elapsed time word problems.

So last week I paired (and three-d) everybody up and we played The Amazing Race...Elapsed Time version! I posed 7 different problems around the room with a basket under each problem.

The students were given 45 minutes to race through and complete as many of the problems as possible...but!! They had to leave their work behind in the basket and they had to make sure that every member of their group understood the strategy they used to solve the problem before they moved onto the next one.

My plan was to gather all of the solutions left behind and be able to assess them for my student's current understanding of how to solve elapsed time problems. What I found was that many (too many) of the groups were doing stacked subtraction and borrowing to attempt to find elapsed time. For example:

10:25

-2:57

Obviously this won't work because time doesn't have the same place value as most of the other numbers we worked with. So I put an example problem on the board using the exact numbers I have displayed above. I showed the answer using stacked subtraction and a second solution using a number line.

I once again split of students into groups and asked them to tell me which answer was correct and why....thankfully, this time the majority seemed to see that the stacked subtraction just wasn't working. We discussed place value and what it means to borrow and why we can't borrow 60 minutes across a column of numbers in a time question. 

Each group was asked to come up with another strategy for solving this same problem and I collected their answers and created an anchor chart (which I didn't take a picture of...sorry!)

So then I sent them out to look over the answers from the previous day's amazing race. I had posted them up together with the correct answers on one side and the in correct on the other. Each student was given 2 post-it notes. The pink one was for them to share what they liked about how the group solved a problem and the yellow was for a suggestion of how the problem could be solved differently in order to find the correct answer. 

And that was that...I asked each of the students to pick 2 activities they do during the day and find the elapsed time between them using 2 different strategies in their math journals....here's hoping they didn't use stacked subtraction!!!