To save fuel on the 240km trip to their cottage, the Nakamura family reduce their usual average speed by 20km/h. This lengthens the journey time by 1 hour. What is the slower average speed?

All your questions are similar, so am helping with only this one, you should be able to work out others.

Suppose lower speed is x

Then usual speed is x + 20

Change in time : 1 hour, time = distance / speed

Hence 240/x - 240/(x + 20) = 1

Solve this for x