Adding and Subtracting Units of Time
Date: 12/11/98 at 10:40:25 From: Shellie Subject: Adding and subtracting units of time How do you solve subtracting units of time? For example, how do you do: 15 hours 52 minutes - 12 hours 85 minutes ---------------------- I thought that the answer would be 3 hours and 7 minutes because you can't do 52 - 85, so I thought you would carry, and that would make it 12 - 5. Since you left the 5 as 4, you have to carry, and the 4 will be a 14, and 14 - 8 is 6. Finally, you have 4 - 2 = 2. So that's how you get 2 hours 67 mins, but then you have to regroup. Since 60 mins = 1 hour, you add another hour to 2 to get 3 with 7 mins left over. My teacher said that my answer was wrong. How do you do it the right way?
Date: 12/11/98 at 12:45:50 From: Doctor Rick Subject: Re: Adding and subtracting units of time Hi, Shellie. First, I'm wondering why one of the times in your example is 12 hr 85 min because this could be better written as 13 hr 25 min (replacing 60 minutes by 1 hour). But I'll do the example as you have it so you can see what is different. You are correct that you can use borrowing (you use the word "carry" but I use that word only when I'm adding). But times aren't in base 10, so you have to think carefully about what borrowing means. Times are in base 60 (which is what the ancient Babylonians used for all their math). Let's not go digit by digit, but look at the minutes as a whole. You are correct that you can't subtract 85 from 52, so you must borrow. How do we borrow? We go to the Hours' house and ask for an hour. Then we change it into minutes: 60 minutes, to be precise. Now we have 52 + 60 = 112 minutes (and 15 - 1 = 14 hours). We can subtract 112 - 85 = 27 (this much is in base 10, so you can borrow in the normal way). Moving on to the hours, we just do the subtraction: 14 - 12 = 2. The answer is 2 hr 27 min. Do you see what you did wrong? When you borrowed an hour, you used the wrong exchange rate - you changed the hour into 100 minutes (10 tens of minutes) instead of 60. That's why you ended up with 40 minutes too many. I hope this helps you. One nice thing about adding and subtracting times is that it can give you a better understanding of what carrying and borrowing is all about. Just remember this: - Borrowing and carrying in regular arithmetic mean exchanging 10 ones for a ten (or 10 tens for a hundred, etc.). - Borrowing and carrying in time arithmetic mean exchanging 60 minutes for an hour. - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
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