learning effect

Skilled Limited works on technically difficult jobs, and as such experiences substantial “learning”. In the past, it has been very unscientific in estimating the effect of the learning, but is now keen to understand how much learning is taking place. Consequently, it has gathered some basic data: the time taken for the first 20 units of a new job was 80 hours but the next 10 units took only 30 hours and the 10 after that took 25 hours. Estimate the learning effect using all the data at your disposal.

my answer was 50%
but in the solution its given 84.375%
pls help