Try this one.
Over a timed 2 mile course, which is uphill half the way and you can only go 15 mph uphill, how fast would you have to go on the downhill side to average 30mph ?
45?
To average 30 mph, you would have to cover the 2-mile course in 4 minutes. At 15 mph, you would take the entire 4 minutes to travel the first mile; therefore you would have to cover the second (downhill) mile instantaneously.
Teleportation, anyone?
Zeko
The last mile is straight down off a cliff, way below the elevation at which you started?
You cannot average 30 mph under the conditions posed by the problem.
Try this one.
How many feet of rope is needed to circle the circumference of a globe the same size as earth?
How many additional feet of rope is required to raise the rope one foot above the globe along its entire circumference?