We know the following about pressure and temperature:

• When an aircraft is in a lower pressure area, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a higher-pressure area, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
• When an aircraft is in a colder airmass than ISA, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a warmer airmass than ISA, the altimeter will under read. The aircrafts true altitude will be higher than indicated.

During the departure we climbed through the transition altitude (18 000 feet) and we switched from a local QNH setting of 1023 to a standard setting of 1013,25.
We return back to the aerodrome but at the transition level (FL190) we forget to switch back to the 1023 pressure setting on the altimeter.

So, we will be landing with the altimeter still set to 1013 instead of 1023.
Because we have a lower pressure set than actual, we already know that the altimeter will under read.
True altitude will be higher than indicated (going from low to higher pressure).
We know 1 hPa represents 27 feet near the ground so:

1023 – 1013 = 10 hPa
X 27 = 270 feet

So, when we land the altimeter will read 270 feet less than the elevation (323 ft) of the aerodrome.
323 – 270 = 53 feet indicated altitude after landing with 1013 still set on the altimeter.

Note: According to the new syllabus (EASA 2020) a barometric lapse rate of 30 ft /hPa should be used (instead of 27 ft/hPa). However, if the question text indicates a different lapse rate, that is the one you are expected to use.