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