The first batch of 50 units will take 750 hours to produce. The total time for the first 16 batches of units was 8500 hours, what is the learning rate?
It is r^4 (i.e. r to the power of 4) that is equal to 0.7803 (and I do explain this in my lectures!)
Therefore r is the fourth root of 0.7083.
You can get this by taking the square root twice, but better is to make sure you have a scientific calculator and can then take the fourth root directly.