Mentor: Consider this problem: "We started our math lesson at 10 a.m., and I expect it to last for 2
hours. When will it end?"
Student: 12 p.m.
Mentor: Great job! Now let's think about our vocabulary words. What do we call 10 a.m.?
Student: That's our starting time.
Mentor: Right, so what is the 2 hours?
elapsed time! We learned that in the last lesson.
Mentor: Exactly. So what do we call the time we were trying to figure out?
Student: 12 p.m.? That's the ending time.
Mentor: Great! How do you think we calculate ending time?
Student: Probably like we calculate elapsed time. Either we could think of it as a regular addition
problem, being careful not to have more than 60 minutes. Or we could split the elapsed time
into groups, first adding the hours, then the minutes.
Mentor: What do you do if you get more than 60 minutes?
Student: When you get to 60, you add another hour and set the minutes at zero, and then keep adding
from there. So when you have 75 minutes, that's the same as 1 hour 15 minutes.
Mentor: Great job! Now you're ready for some really tricky problems.