QFF is effectively the pressure measured at a specific datum (airfield, meteorological station etc) adjusted to sea level using the actual conditions occurring at the time.
If you had the QFF set and landed on the sea your altimeter would read zero.

QNH is the pressure measured at a specific datum (airfield, meteorological station etc) adjusted to sea level using the lapse rates etc of the International Standard Atmosphere.
If you had the QNH set then at the airfield your altimeter would read the elevation of the airfield and if you landed on the sea it would only read zero if the actual conditions were the same as the ISA conditions ( that is if QNH=QFF).

Because the airport is situated 400 meters above sea level with a QFF of 1016 and the question states the air temperature is 10 degrees lower than ISA, we can calculate the QNH.

400m x 3,285 = 1314 feet
X 1,04 (4 % temperature correction for every 10 degrees below ISA) = 1366,6 feet
/ 27 (drop of 1 hPa according to ISA near the ground) = 50,6 hPa

1314 feet / 27 = 48,7 hPa

50,6 -48,7 = roughly 2 hPa. So, the QNH will be 1014 and the QFF = 1016.

We do not need to do these time-consuming calculations because most of the questions just ask if the QNH would be more or less than QFF.

Actual pressure at sea level = 1016 hPa, because the temperature is colder than ISA, the air is more dense and thus pressure will drop faster with altitude.
QNH takes into account the International Standard Atmosphere average drop of 1hPa with 27 feet.
Because at an airfield we still want to read the correct elevation above mean sea level of the aerodrome, the QNH has to be adjusted.
The QNH in this case needs to be lowered (lower than 1016) because if you would set 1016 the temperature factor has not been taken into account and thus the altimeter will overread.

Remember:

ABOVE MSL:

• Temp warmer than ISA => QFF < QNH
• Temp colder than ISA => QFF > QNH

• Temp warmer than ISA => QFF > QNH
• Temp colder than ISA => QFF < QNH