Kraggy and to the pilot(s) who expressed his concern
I think one reason why the answers above are diverse or unspecific is that this is a poorly worded question, so badly worded it is unanswerable without making assumptions. I agree with ATCO 2 (although I was taught to use 1mb = 30' not 28'... I think for simplicity?). Placing neck & reputation on the block...
If you take off from A (elev 500'amsl) and climb to a height of 500' (height = distance above a datum, and in this case I will assume the general usage that the datum in question is the airfield?). Height + elevation means you will have climbed to 1000'amsl.
You fly straight and level and keep 990mbs set on your altimeter until you arrive at B. Here the QNH is 1010mbs, so your vertical position is not being measured above amsl, but from the point in space at which the atmospheric pressure is 990mbs.
The difference between QNH(A) and QNH(B) is 20mbs, which represents a vertical distance of 600' (20 X 30 = 600). Remembering that pressure decreases as you rise through the atmosphere, that point will be 600' above sea level, thus your true altitude will be 1000 + 600 = 1600'.
With an airfield elevation of 200', your height above Runway B will be 1400'
Have I passed?