For part a) of this question, I calculated the revised time per unit of 2.08 by subtracting the total time for 999 units from the total time for 1000 units and then multiplying this time by 460 units to arrive at the revised time. But this seems to be wrong I don’t understand why.
They only actually produced 460 units and therefore we only need to use the learning formula to calculate what the total time should have been for 460 units.
The only relevance of the mention of 1,000 units is that because they produced less than 1,000 then the learning rate applied to all 460 units that were actually produced. What you were doing would only have been relevant if they had already produced 1,000 units, but this is not the case – they produce 460 and these are the first units that they are producing.