A 10 kVA transformer has an iron loss of 100 W and a full-load copper loss of 400 W. Find (a) the load fraction (as a decimal of full load) at which maximum efficiency occurs, and (b) the maximum efficiency at PF 0.8 in %. Over a day it runs 8 h at half load (PF 0.8), 8 h at quarter load (PF 1) and 8 h on no load. Find (c) the all-day efficiency in %.