Furthermore I have a simpler question of a different ty[pe that I can't do.

This is the question:

A car must complete a journey at an average speed of 40mph. At what speed must it travel on the return journey if the average speed for the complete journey is 60mph?

I started off:

Average speed =

Then because T1 =

Average speed =

To which I think I can cancel the 2D's. I'm left as T2 = 20mph but that's not the answer.