Nav Computer: In AIRSPEED window put temperature (-15ºC) next to MachNo Index.  

Go to TAS (250kts) on inner scale and read Mach number next to it on the outer scale: MN 0.4.

Calculator:  MN = TAS/LSS = 250/38.94 √ (273 – 15) = 250/38.94x √258

= 250/38.94x16.06 = 250/625.5 = 0.4