To solve this problem you must use the equation for the relationship between SAT and TAT. This is:
SAT = (TAT / (1 +(0.2 x k x Mach no squared))
This can be rearranged to give:
Mach no = Square root of ( (TAT/ (k x SAT) )-1 ) / 0.2)
k is the ram recovery factor of the temp probe, which is one for a perfect probe. If we assume that k is 1 we can ignore it in the above equation.
We must be careful to note that SAT and TAT must be in degrees absolute.
At 40000 ft in the ISA SAT is approximately -56.6. Adding 273 to this gives 216.5 absolute.
The question states that TAT is -29. Adding 273 to this gives TAT = 244 absolute.
Putting these values into the above equation gives:
Mach no = Square root ((244/216.5) -1) / 0.2)
This simplifies to give mach 0.7969 or approximately Mach 0.8
It is probably possible to do the above on A CRP5 but I have not tried to do so.
The process probably involve working out the ram rise, which is TAT - SAT. Then using this in the CRP5 to get the Mach number that would give you that rise.