leaning effect

The times taken to produce each of the first four batches of a new product were as follows: Batch number Time taken 1 100 minutes 2 70 minutes 3 59 minutes 4 55 minutes What was the rate of learning closest to? i know there is a different approach to this question by 100r2 = (100 + 70 + 59 + 55)/4, giving r = 84.3% but first cross my mind to d o this 100+70=170/2=85 100+70+59+55=284/4=71 71/85=83.53 i don't understand isn't that the correct way as well ? why should i take The overall rate of learning is an average of these two rates of learning, so we have (85% + 83.53%)/2 = 84.265, say 84.3% that doesn't make sense to me if start from the begging " when output is doubled accumulated average time reduce by the learning rate " if i applied this it doesn't work out correctly .when i doubled to 2 the LR was 85 and when i double 2 the learning effect suppose to be the same , same 85 when i tried to take the percentage of 84.265 doesn't come to 71 but 83.53 does