Learning rate. Kaplan question 121

Dear John, I am struggling with the below question. " The time for the first batch of 50 units was 400 hours but the labour budget is the subject to a learning effect where the learning rate is 90%. The rate of pay for labour is $12 per hour. The business had received and satisfied an another order for 600 units but is now received a second order for another 800 units. The value of b i= -0.152 What will be the cost of this second order? no decimal please" It appear to me that the learning process has stooped, hence we have successfully completed 600 units. I though we should you the equation Y=axb, to calculate average time per 800 units and them multiply results by 800 to get total hours for 800 units. Finally the total numbers of hours times rate of pay so then Y= 400 hours x 800 units to (-0.152) etc. But the Kaplan answer is different. How would you attempt this question? *I have watched the free lecture on this subject many thanks Anna