Finding the end time for a movie in 24 hour time | Math | Khan Academy

We are told that Andre goes to a movie that starts at 19 hours 45 minutes, or 1945, and is 90 minutes long. What time is the movie finished? So pause this video and see if you can answer that before we work through it together.

All right, so what I like to do is set up what we could call a timeline. So if this is our timeline, and let's say that this is 19 hours right over here, 19 hours, maybe this is 20 hours right over there, and then this would be 21 hours right over there.

Now what time does the movie start? It starts at 19 hours 45 minutes, so this would be 19 hours 30 minutes, which would be halfway between 19 hours and 20 hours, and 19 hours 45 minutes would be halfway between 1930 and 20. So 19 hours and 45 would be right over there.

So let me write that down. That is 19 hours and 45 minutes, and that is when this movie is going to start. Now it's going to continue for 90 minutes. So when does it end?

So there's a couple of ways you could think about it. The way I think about it is it will take us 15 minutes to get to 20 hours, so that's 15 minutes, and so 15 minutes of the movie has happened by 20 hours.

Then how much is left? Well, 90 minus 15 is 75, so then there's another 75 minutes left. Well, 60 of those 75 are going to get you to the next hour, so then it's going to be like this.

So then you're going to have another 60 minutes to get to the next hour. So so far we've used 75, and then you're going to need another 15 minutes to have a total of 90 minutes.

So then you're going to have another 15 minutes. When does the movie end? Well, as you can see, we started in 1945. 15 minutes got us to 20. Another 60 minutes got us to 21, so now we have a total of 75, and then another 15 would give us a total of 90.

So we are finishing 15 minutes after the 21st hour, or after hour 21, or however you want to think about it. So this time right over here is going to be 21 hours and 15 minutes, or 21:15. So that's the time that the movie is finished.