Welcome to our lesson on calculating speed as a compound measure of distance and time. So we have this formula here where speed is distance divided by time and in question one we're told that a car travels a total distance of 450 kilometers under a time of five hours. So we have the speed is equal to the distance which is 450 kilometers It's important to include the units divided by the time of 5 hours and we can work out the 450 divided by 5 which is 90 and the kilometres divided by hours which we write as kilometres per hour.
On the question 2. The distance between New York and California is 2,900 miles. The plane takes off from New York at 8am and lands at California at 12.30pm the same day and we're asked to calculate the average speed of the plane. So we're going to begin by working out the time and the time is the difference between 8am and 12.30pm which is four and a half hours.
which we'll write as 4.5. So this is the 4 hours and 30 minutes which is half an hour and the distance we are told is 2,900 miles. So again the speed is equal to the distance 2,900 miles divided by the time of 4.5. hours.
So the average speed is approximately 644.4 miles per hour. Okay, let's try some more questions. So in question three, we're told a van travels 160 kilometers at 40 kilometers per hour, and we're asked to work out how long the journey takes. So we're going to substitute these values into our formula where the speed is 40 kilometers per hour and this is the distance of 160 kilometers divided by the time which we're trying to find. So we'll move the time to the left hand side.
So the time multiplied by 40 is equal. to 160. We divide both sides by 40 so the time is 160 divided by 40. We can simplify this fraction by dividing both by 10 and 16 divided by 4 is 4. So the total time is 4 hours. So for question 4 I'm going to draw a diagram. and I'm going to put the diagram up into two parts.
The first part of the journey, a boy walks for five kilometers at a speed of eight kilometers per hour and I'll put this here so five kilometers at eight kilometers per hour so I have the distance, I have the speed, I do not have the time and in the next part of our journey He cycles 6 kilometers at a speed of 12. So now the distance is 6 and the speed is 12. And again I do not know the time. So I can work out the time for the first part of the journey using our formula where the speed of 8 is equal to the distance of 5 divided by time. So the time will be 5. divided by 8 which is 0.625 of an hour.
I'll come back to that in a second. For the second part of our journey the speed of 12 is equal to the distance which is 6 divided by the time. So the time here will be 6 divided by 12 which is 0.5. We'll add these two times together.
So the time in total is 1.125 of an hour. And to work out the average speed of the entire journey we need the total distance which is nis 5 at nis 6 which is 11. So the speed is equal to the total distance of 11 divided by the total time of 1.125. So the speed is 8.8 kilometers per hour.
Okay, thank you for watching and I hope you found that useful. Thanks again and take care.