by tommywallach Sun Mar 10, 2013 9:13 pm
Hey Momo,
They don't actually do anything with it, so I wouldn't worry about it. If you're asking why they placed it there, it's merely to remind people that you can't add either rates or times, but you can add distances. However, in this example, we don't need to do anything with that information, and the equations reflect that.
Hope that helps!
-t