For part a) of this question, it is mentioned in the book that a is the cost for the first batch instead of the time taken for the first batch. So is it correct.
Also, I got the profit figure of 22395278.2437236 instead of the profit figure given in the book of 22396225. So is it correct.
If you use a as the time for the first batch, then the answer using the doubling rule or the formula is the time. If you use a as the cost for the first batch, then the answer using the doubling rule or the formula is the cost. It make absolutely no difference which you use.
Of course your answer is correct. It is the same as in the book apart from rounding differences, and rounding differences are irrelevant just as they are irrelevant in real life.