You need to be at a true altitude of 15,000ft above the QNH datum,1003hPa.  

The difference between standard pressure (1013hPa) and QNH (1003hPa) is 10hPa and at 30ft/hPa this is equivalent to 300ft.  

This means that we need to be at 15,300ft above the 1013hPa pressure datum.  

The difference between pressure altitude and true altitude is 4ft per thousand per degree of temperature deviation; 4 x 15.3 x -15 = 918ft.  

We need to be at an indicated pressure altitude of 15,270 + 942 = 16,218ft