The key information to find out the answer is: the temperature given is TAT (that is also know as RAT or IOAT) or SAT (COAT or simply OAT).
I understand that it's referring to SAT, then TAT = SAT + Ram rise, TAT = -24 + 20, TAT = +4,
therefore 262 cas is 500 TAS, correct for compressibility and you will get 479kts
Correct me if I'm wrong.
Cheers