Even i was caught in the trap of answer A.
I am always cautious while marking (A) as my answer. Not because the probability of it being the answer is low, but because if the question has a trap people could fall for, then the question maker will set (A) as that particular lucrative option. (Hell, I do that when I make questions! Kind of a bait!) So I like to double check especially if it seems too easy.
The logic of (555 - x)/y was perfectly valid. The only missing point was that these are the (n -1) additional minutes. You did not account for the 1st minute. So total time will be (555 - x)/y + 1 which is the same as option (C): (555 - x + y)/y
And yes, parentheses will definitely be there in the exam in such a question. 555 - x + y/y = 555 - x + 1 (Since it didn't make any sense here, it was obvious that there are parentheses in the options). Rest assured, there will not be any ambiguity in the actual exam.
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