Merci blackmail, I thought this was more interesting as it relies on the knowledge that a constant CAS and flight level produces a Mach number that is independent of temperature. There is no need to assume ISA, any temperature will do, then it can be very quickly calculated on the nav computer without using formulae.
Using your last equation I get an answer of M0.419, the nav computer gives M0.445. The multi-choice answers above were made up, if this question comes up again I'd be interested to know what they are.
(edited to add: my mistake, I am getting M0.420 now so there is no real discrepancy)
Last edited by Alex Whittingham; 8th Dec 2014 at 09:06.