1033 – 1013 = 20hPa x 27 = 540ft Altitude in ISA = 20,540ft

FL200 ISA = +15º - (2º x 20) = -25ºC

OAT = -40ºC

Deviation = -15ºC

Temperature error = 4 x 20.5 x 15 = 1,230ft

True altitude = 20,540 – 1,230 = 19,310ft