A 100 kva, 3-phas, 50 Hz, 3300/400 v transformer is delta connected on the high voltage (h.v) side and y-connected on the low voltage (l.v) side. the resistance of the h.v winding is 3.5 Ω per phase and that of the l.v winding is 0.02 ohm per phase. calculate the core loss of the transformer if its full-load efficiency is 95.8 % at 0.8 pf lagging.