Time differences | Math | 3rd grade | Khan Academy

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 has passed since the clock said, "Read this first time," till 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, sometime on Monday, and the second one was sometime on Wednesday.

And these hands have been going around and around. We know the time passed is less than 12 hours. So, the first thing I think we should do to figure 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 past six o'clock, but it's not quite to seven. So, it's six something. It could be 6:15, 6:20. We'll figure out the minutes next, but we know it's some amount of minutes after six but not yet seven.

Now let's look at those minutes. The minute hand, again, started at the top. It's gone five minutes, ten, fifteen, twenty, twenty-five, thirty, thirty-five, forty. Can't go to forty-five here, so we 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 ten yet. Then our minute hand right here starts at the top and it's five, ten, eleven 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 just see how long it takes to get to seven o'clock. So, on 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, ten, fifteen more minutes to get all the way to the top of the clock.

So, until seven o'clock will be two minutes plus fifteen 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 around another hour to be nine o'clock. So that's a total from seven to nine of two hours.

Then finally, from nine until 11 minutes after nine, the minute hand will be at the top, and it'll go five, ten, eleven minutes. So, this will be 11 more minutes.

In total, from 6:43 all the way to 9:11 is 17 minutes plus 2 more hours plus 11 minutes. So, we can write that as going to be two hours, and then we can combine our minutes: 17 and 11.

Seventeen and eleven, that will be eight ones, a ten and a ten is twenty, 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.