Hopefully the previous posts have solved your problem. but if not the following may help.
Q - You are setting your altimeter at an aerodrome, where the elevation is 18ft. The QHN you have been given is 1005mb. What would you expect the aerodrome QFE to be?
A) 1006mb B) 1003mb C)1004mb D)999mb
QNH is the pressure at mean sea level.
QFE is the pressure at the surface of the airfield.
Pressure decreases at a rate of about 1 mb (or Hpa) per 30 feet altitude increases.
Elevation is measured from mean sea level.
In this question the airfield is 18 feet above mean sea level so the pressure at the field will be about (0.5mb less than that at mean sea level.
With QNH of 1005 mb the QFE (pressure at the airfield) must be about 0.5 mb lower, which is 1004.5 mb. So the best option is 1004 mb.
Q- You are flying at FL55, what is the altitude of your aircraft if the QNH is 1008mb?
A) 5200ft B) 5350ft C) 5650ft D) 5750ft
Flight levels are measured in hundreds of feet above the 1013 mb level. So FL55 is 5500 feet above the 1013 mb level.
QNH (the pressure at mean sea level) is stated to be 1008 mb, so mean sea level must be higher than the 1013 mb level.
Subtracting 1008 from 1013 gives a difference of 5 mb. Using the standard pressure lapse rate of 30 per mb this equates to 350 feet.
So mean sea level is 150 feet above the 1013 mb level. Subtracting 150 feet from 5500 feet give an altitude of 5350 feet above mean sea level.