The learning rate applies to the cumulative average unit time every time cumulative output doubles.. So, as cumulative output goes 1, 2, 4. 8, cumulative average unit time will go
100 100 x 0.75 = 75 75 x 0.75 = 56.25 56.25 x 0.75 = 42.1875
If the cumulative average unit time for 8 is 42.1875, the total time for the 8 units is 8 x 42.1875 = 337.5.