From 15:00 to 13:38 = 1hr 22min = 82min (82/60=) 1.367hr

1. Groundspeed?

GS=Dist/Time = 570/1.367 = 417kt

2. TAS?

GS 417 and HWC 52kt = TAS 469kt

Now on the CRP-5 put -48 under mach number index (in Airspeed window), put TAS (outer scale) and find the reduced mach nr. (inner scale) = 0.80