Thanks a lot.
I know the question is crap but that's what is given to me, word to word. The solution uses the CRP5 to answer it, unfortunately, I'm lacking one.
The ,correct' formula is this one, by the way:
TAT= SAT(1+0,2Mē)
I know about this formula but ironically I need the SAT to calculate Mach speed. I avoided buying a CRP5 because of the high price and got an E6B instead (less than a third of the price), sure it would do the job since all the guys in the US seem to use it with no problem.
I find myself in an annoying situation. I'll end up spending more. I'll try using the other formula you suggested, thanks a lot!