AAT CBT sample assessment 2 - costs and revenues

Can anyone shed any light on this?
Having completed the costs and revenues sample assessment 2 on the AAT website, I am going back through and checking my answers against the model answers provided. There is a mistake on their answer sheet (I think) for Task 3.
For the calculations, I had a) £9900, b) £3744, c) £2088 and d) £15732. The answer sheet says different but I think there is a mistake which lies in either their answer b) or d).
Did anyone else spot it and do you know which answer is incorrect?
Having completed the costs and revenues sample assessment 2 on the AAT website, I am going back through and checking my answers against the model answers provided. There is a mistake on their answer sheet (I think) for Task 3.
For the calculations, I had a) £9900, b) £3744, c) £2088 and d) £15732. The answer sheet says different but I think there is a mistake which lies in either their answer b) or d).
Did anyone else spot it and do you know which answer is incorrect?
Comments
Your answer b) is incorrect.
The question ask for the labour cost PREMIUM of time and a half. This differs from c) where it asked for TOTAL labour cost of double time working. The overtime premium is only concerned with the "half" in the time and a half (not £18 which is the total), only the £6.
(The distinction is to do with the premium being considered indirect labour cost)
d) ask for TOTAL labour cost so your answer is correct.