I will answer you, but in future you must ask in the Ask the Tutor Forum if you want me to answer – this forum is for students to help each other 🙂
The return per hour = throughout per unit / hours per unit.
The throughput return = 320 – 80 = 240 The hours per unit = 6/60 = 0.1
Therefore the return per hour = 240 / 0.1 = 2,400.
I do suggest that you watch my free lectures on throughput accounting.
Also, I have uploaded lectures working through the whole of the June 2015 exam (and other exams). You can find them linked from the main F5 page as “F5 Revision Kit Live”