The exam as a whole is 100 marks and there are 3 hours (180 minutes). So you should aim to spend about 1.8 minutes per mark.
Since section A is 40 marks in total, you should spend no more than 72 minutes. Similarly for section B, you should be aiming for 18 minutes each for the 10 mark questions, and 27 minutes each for the 15 mark questions.