GRE math questions on TestBig

I have a few doubts regarding the answers to your questions on GRE Math practice:

Question 3:
Say the number of commuters in June were 100, if they decreased by 15%, then in July, there were 85 commuters. Reduced by a further 10%, there were 85 - 8.5 = 76.5 (theoretically of course!) commuters in August. Then, to equal the number of commuters that they had in June, in September, wouldn't the % increase be 24% (Option E)? please tell me where I'm going wrong in this sum!!


Question 5:

I solved it like this: I assumed that the say the total cost of labour and parts is equal to 100, then the cost of labour is 75 and the cost of parts is 25. After reduction due to technological advances, the present costs would be 45 for labour and 12.5 for parts, making a total of 57.5. So isn't the % decrease actually 100 - 57.5 = 42.5, instead of 57.5? Where am I going wrong in solving this sum?