A person goes to his friends home at the speed of 30 km/h and comes back at the speed of 40 km/h. what was his average speed throughout the entire trip?

Let be the distance between the two houses. Considering the formula

where is the distance, is the velocity and is the time, we can solve it for the time to get

So, the two trips take the following time:

To get the average speed, we need to consider the two trips together. Globally, the person traveled a distance of , and the global time is the sum of the two times. We have

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