I agree with Old Smokey that, in my experience, some ATPL exam questions are overly pedantic but usually to avoid ambiguity the conversion factors to be used for a particular question will be stated either in the instructions at the front of the exam paper or in the question itself. My advice, read the questions very carefully. If the worst comes to the worst and you believe the question to ambiguous, you have the option to challenge the examining authority (which I have seen work on more than one occasion).