The Mach Number = TAS/LSS(local speed of sound)
Local Speed of sound = constant(38.94) x square root of temp(in degrees kelvin)
Just transpose the 2 formulae above.
And remember that the speed of sound is dependent on temperature.
Years since i did that in ATPL studies.....frightening thats its still in my head!!
And having just rattled it out on the calculator i get 250 knots!