The CRP5 solution:
TAT is -29, ISA is -56.5, temperature rise is 27.5º
On the CRP5 look at the blue temperature rise scale, against 27.5º read off a TAS of 500KT. Set the Mach number index in the AIRSPEED window to -56.5º. Against the outer scale TAS of 500KT read off the Mach number on the inner scale of M0.88.
I guess the answer is different because the blue temperature rise scale uses a recovery factor that is less than 1.0. I don't know what the factor is or whether it is typical of a modern system.
We are not aware of any CQB questions like this.