Acca December 14 Section b Question 1 part a

Hi Sir hope your well i have another query which i was hoping you would be able to answer.
With regards to Question 1 part a section b of the December 14 paper.
Its asked us to calculate the price which is expected to be charged for the new seat with the labour cost being based on the time taken to produce the 8th chair.
Now the approach i took was
i done total time taken to produce 8 units less total time taken to produce 7 units, giving me the time taken to produce the 8th unit. my calculations were as follows.
total time taken to produce 8 units: Y=12×8 to the power of -0.415 multiplied into 8 units=40.503158 hours
less total time taken to produce 7 units= Y=12×7 to the power of -0.415 multiplied into 7 units=37.45964 giving me time taken to produce 8th unit=3.043518 hours.
i then done 230+(3.043518 hours x $15 per hour)=$230+$45.65=$275.65 x 0.5=137.83+275.65=$413.48 as my final answer.
My final answer is correct but once i see the mark allocations its said that you get 1 mark each for working out cumulative average time per unit for 8 and 7 units. but i didnt press the = sign i multiplied it into 8 and 7 units to get the total time as shown above does this mean even though my final answer is correct according to the workings shown above i still wont get the 5/5 marks for part a question 1?
Thanks once again:)