overyourhead

Brain teaser answer...
240mph

Distance = speed x time

Average speed over = s1 = 696
Average speed back = s2 = 145

So if time over is t1 and time back is t2 then distance = s1 x t1 = s2 x t2

Total average speed = (2 x distance)/(t1 +t2) = (2 x s2 x t2)/(t1 + t2) = (2 x s2)/(t1/t2 + 1) = (2 x s2)/(s2 / s1 + 1) = (2 x s2 x s1)/(s2 + s1) = (2 x 145 x 696)/(145 + 696) = 240