On the whizz wheel:
In the ALTITUDE window put pressure altitude (20,000ft) next to
temperature (0ºC). Go to indicated altitude on the inner scale (19,300ft) and
read true altitude on the outer scale (21,200ft).
ISA Temperature at FL200:
15 + (20 × -2) = -25°C
As OAT = 0°C; the air mass is ISA + 25°C.
The question states 'When the actual air pressure on an airfield at MSL
is placed in the subscale of the altimeter the indicated altitude is 19300ft';
so the barometric altitude (QNH) is 19,300 ft, as the airfield is at MSL.
Therefore all that the question requires is to apply a correction for
temperature error to the given barometric altitude, to find the aircraft's true
altitude.
Temperature correction:
19.3 × 4 × (+25) = +1930 ft
Now all we have left to do is apply the temperature correction to the
barometric altitude.
True altitude:
19300 + (+1930) = 21230 ft.
Given answer is 21,200 ft.
Meteorology Chapter 1 (1.12 to 1.18) goes through the process step by
step.