There are 2 considerations in converting Indicated Altitude to True Altitude. These are Barometric Error and Temperature Error. The question tells you that there isn't any Barometric Error. You have actually set to the QNH at the station. (The numerical value of the QNH is a red herring in this question).
However, QNH uses the ISA atmosphere for conversion to sea level (because that's what your altimeter uses, and you want to see the correct altitude of 2500 when landing, not the temperature-corrected altitude). So you are only measuring the temperature difference in the column of air between the airfield and 10,000 feet, ie a column of air of 7500 feet.
The temperature correction formula is:
True Alt = Ind Alt + (ISA dev x 4/1000 x Ind Alt)
In this case, substitude 7500 for 10000 feet in the second "Ind Alt" in the above.
So True Alt = 10000 + (-15 x 4/1000 x 7500) = 9550
Last edited by oxford blue; 13th Feb 2003 at 18:45.