Finding out learning rate

Dear John,
In free lectures, we were taught about the time taken to produce XX units. And we learnt that. However, while doing the BPP kit questions, i came across to the question where it asked to calculate learning rate. As far as i remember, the calculation of learning rate is not covered in the lectures. One of the kit question is given below along with answer. Can you help me understand that because i am not able to understand this answer.

SCENERIO:
Kiss Co has developed a new product. The first batch of 200 units will take 3,500 labour hours to produce. There will be a 75% learning curve that will continue until 4,800 units have been produced. Batches after this level will each take the same amount of time as the 24th batch. The batch size will always be 200 units.
Note. The learning index for a 75% learning curve is –0.415

QUESTION:
The total time for the first 16 batches of units was 22,000 hours. What was the actual learning rate, to the nearest %?

ANSWER:

Batches Hours
1 3,500
2 Doubled once
4 Doubled twice
8 Doubled three times
16 Doubled four times 22,000
Cumulative average time per batch = 22,000 / 16 = 1,375
1,375 = 3,500 r4
r = 0.79 or 79%

I AM HAVING A DIFFICULTY IN RESOLUTION OF r4. HOW IT IS RESOLVED IN THIS QUESTION, PLESAE GUIDE