When answering the question in object I calculate the time to produce the 1000th unit and then multiplied with the actual production 460 units. As per answer it was wrong.
By the time they are producing 1,000 units, they will be producing each unit faster then when they are producing the first 460 units.
The time to produce 460 units is 460 x the average time per unit when producing 460. The average time per unit to produce 460 you get using the formula.
The free lectures on learning curves will help you.