How to calculate aerodrome elevation given QNH and QFE Would someone be able to explain how to get the answer to the following question?

13) The QFE for the aerodrome is currently 985 hectopascals. The QNH you have been given is 1031 hectopascals. Approximately what would you expect the aerodrome elevation to be?
A 1380 ft
B 690 ft
C 2663 ft
D 875 ft

Correct answer is A.

Thanks
QNH is the pressure setting that will show the aerodrome's true altitude at field level, QFE is the pressure setting where the altimeter will show zero feet at field level. The rule of thumb is that pressure changes 1 hpa per 30 feet, so the formula you would use is (QNH - QFE) * 30feet/hpa = field height. In this case that is (1031hpa - 985hpa) * 30feet/hpa = 46hpa * 30feet/hpa = 1380ft. 

If the QFE is greater than QNH you would get a negative number, which would mean the airfield is below sea level. This is pretty rare, but they do exist, so don't mess with the QNH - QFE order.