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.