I guess a good example is a plane making a 100km trip at 100km/h. If the wind is against it from A to B at 50km/h, the plane’s net speed is 50km/h so the one-way trip takes 2 hours. Then on the way back if the plane’s net speed is 150km/h, the 100km return trip will take 0.67 hours resulting in a total trip time of 2.67 hours. Compare this to if there was no wind and the plane flew 100km each way at 100km/h; the trip would take 2 hours.

“Always account for variable change!” -Kevin Spacey, 21