# Elapsed Time

Shodor > Interactivate > Discussions > Elapsed Time

 Mentor: Consider the problem from before: "We started our math lesson at 10 a.m. It is now 11 a.m. How long have we been doing math?" Student: 1 hour. Mentor: Right, and the fancy name for the time that has passed is elapsed time. How do we figure out the elapsed time in a problem like that? Student: I just subtracted: 11-10=1 hour. Mentor: That's a really good method. Look at this problem: "If it's 1 p.m. now and I got to school at 8 a.m., how long have I been at school?" How would solve a problem like that? Student: I would probably just count. 1 hour until 9, 2 hours until 10, 3 hours until 11, 4 hours until 12, 5 hours until 1. 5 hours. Mentor: Those are both really good methods of solving problems, but you can't forget that there are sometimes minutes as well as hours in these problems. Try this one: "If you started eating lunch at 11:45 and finished at 12:05, how much time is that?" Student: I would solve that problem by adding the minutes. 15 minutes to 12:00, and 5 more minutes to 12:05. That's 20 minutes total. Mentor: Great! Can you think of another way to solve it? Student: You could subtract. Mentor: How can you subract. You can't borrow like you normally would. Student: You could take 1 hour and turn it into minutes. That would give you 11 hours and 60+5 minutes, 65. Then you subtract: 65-45=20 minutes. And 11-11=0 hours. So the answer is 20 minutes. Mentor: Great job! Now you're ready for some really tricky problems.