Rearranging your equation
True Altitude = Altitude on QNH + [(ISA Deviation x 4) x (Pressure Altitude/1000)]
Gives
Alt on QNH = True Altitude - [(ISA Dev x 4) x (Pressure Alt/1000)]
Inserting the data provided gives
Alt on QNH = 15000 – ((-15 x 4 x 15) = 15900
But we are going to use 1013 which is 10 HPA higher than QNH.
Using 27 ft / Hpa this means that the 1013 level is 270 feet below msl
So adding 270 to 15900 gives an indicated altitude of 16170 feet.
At that altitude the estimated lapse rate of of 27 feet per Hpa is a bit too low. It looks as if the examiner has used 33 ft per HPa (30 HPa per 1000 ft) which is probably more realistic.