240mph.

Note that we cannon add the speeds together to find the average, so 420.5 mph isn't correct.

If we let the distance be x miles.

Using Time = Distance ÷ Speed:

x x

Outward trip = --- hours. Return trip = --- hours.

696 145

The total distance travelled is 2x.

x x

Total time = --- + --- hours.

696 145

Since Speed = Distance ÷ Time, the average speed is:

2x

--------

x x

--- + ---

696 145

The x's cancel to give

2

--------

1 1

--- + ---

696 145

2

-------

1 ÷ 120

This simplifies to give 240 mph. QED.