Although I will answer you, in future you must ask in the F5 Ask the Tutor Forum and not in the General Forum (otherwise I will likely not notice it!).
You really should watch the free lecture on learning curves – this is nothing to do with the formula, it is testing you on the doubling rule, and I cannot type out the whole lecture here.
The average time per unit if they make 2 units is (8 + 6) / 2 = 7 hours.