learning curve

hi sir,

i am doing a qn from kaplan exam kit the qn is:-

the time for the first batch of 50 units was 400 hours but the labour budget is the subject of 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 order for 600 units but it has now received a second order for another 800 units.

the value of b is= -0.152
what will be the cost of this second order? (no decimals please) $_________

i have done this qn using batches and units seperately seperately..

for the batches i took the 600 units and 800 units and divided it by 50 units respectively and i got 12 and 16 batch, then i used the formula y=ax^b to find the total no of hours of 12th and 11th batch (3290.074165 and 3056.05375 hours) after finding the total no hours of both of them i subtracted them and i got 234.0204152
then to find the total no of second order which is the 800 units (16 batches) i did:-

total for first 12 batches = 3290.074165
next 4 batches (16 batch less 12 batch)
(4*234.0204152) 936.0816608
______________
4226.155826
* $12
______________
$50713.86991
________________

for the units i took the 600 units and 800 units , then i used the formula y=ax^b to find the total no of hours of 600th and 800 th (151.2798208 and 151.3181818 hours) after finding the total no hours of both of them i subtracted them and i got 128.30000000015888818
then to find the total no of second order which is the 800 units (16 batches) i did:-

total for first 600 units = 90,767.89248
next 200units (800-600 units)
(200*128.3015818) 936.0816608
______________
116428.2088
* $12
______________
$1397138888.506
________________
after doing both the ways i could not get the correct answer
i have no idea where i have gone wrong
i am not sure if you are suppose to use the batches or the units..

please kindly guide me sir where i have gone wrong..