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Time differences example

CCSS Math: 3.MD.A.1, 4.MD.A.2

Video transcript

- [Voiceover] How much time has passed from the time on the left to the time on the right? So, we have a clock on the left and a clock on the right with different times shown, and we want to know how much time since the clock said, that read this first time, 'til it read the second time. We're also told the time passed is less than 12 hours. That's helpful. That means we know that this first clock wasn't say some time on Monday and the second was sometime on Wednesday, and these hands have been going around and around. We know that time passed is less than 12 hours. So, the first thing I think we should do to find out the difference in these times is to figure out what are these times. Let's read these clocks. Over here on the left, looking at our hour hand first, it's between six and seven. So, the hour hand starts at the top and it's gone passed six o'clock, but it's not quite to seven. So, it's six something. Could be six fifteen, six twenty, we'll figure out the minutes next. But we know it's some amount of minutes after six, but not yet seven. And now let's look at those minutes. The minute hand, again started at the top, and it's gone five minutes, 10, 15, 20, 25, 30, 35, 40. Can't go to 45 here, so we've got to count by ones. 41, 42, 43! It is 43 minutes after six o'clock, or 6:43 on the left. On the right, our hour hand is just after the nine, so it's soon after nine o'clock, but it's definitely not to 10 yet. And then our minute hand, right here. Start at the top and it's five, 10, 11 minutes after the hour. On the right. So, the clock on the right is 9:11. And now what we want to know is how long does it take to get from this first time, 6:43, all the way to the second time, which is 9:11. One way we could figure this out is to break it into little parts. Let's first, let's first just see how long does it take to get to seven o'clock? So, in this first clock as soon as the minute hand gets up to the top it'll be seven o'clock. Let's see how long is that. From here where it is, it has two more minutes to get to the nine. And then from there it has five, 10, 15 more minutes to get all the way to the top of the clock. So, until seven o'clock it'll be two minutes plus 15 more minutes, or we can write that as 17 minutes. Then from seven, let's stick with the hours for a second. Let's just get from seven to nine o'clock. So, from seven to nine. At seven o'clock the hand is at the top, it'll go all the way around one hour to be eight o'clock. All the way another hour to be nine o'clock. So that's a total, from seven to nine, of two hours. And then finally from nine to 11 minutes after nine. The minute hand will be at the top and it'll go five, 10, 11 minutes. So, this will be 11 more minutes. So, in total, from 6:43 all the way to 9:11 is 17 minutes, plus two more hours, plus 11 minutes. So, we could write that, that's going to be two hours, and then we can combine our minutes, 17 and 11. 17 and 11. That'll be eight ones, a 10 and a 10 is 20. So, 28 minutes. It takes two hours and 28 minutes for the clock to go from 6:43, this time on the left, until 9:11, the time on the right.