The question states that the current altimeter setting of 1013.2 will not be changed upon landing to the local QNH of 1023. This means that a lower pressure value is set on the altimeter. If we set the QNH of 1023 hPa on the altimeter the altimeter would read the airports elevation of 290 ft.
1023 – 1013 = 10 hPa
10 x 27 = 270 feet
We now need to subtract 270 ft from 290 ft because we go to a lower pressure value set on our altimeter subscale. 290 ft - 270 ft = 20 ft will be indicated on the ground with 1013.2 hPa still set.
Note: According to the new syllabus (EASA 2020) a barometric lapse rate of 30 ft /hPa should be used. However, if the question text indicates a different lapse rate, that is the one you are expected to use.
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