A single-phase transformer is rated at 10 kVA, 230V/100V. When the secondary terminals are open- circuited and the primary winding is supplied at normal voltage (230V), the current input is 2.6A at a power factor of 0.3. When the secondary terminals are short-circuited, a voltage of 18V applied to the primary causes the full-load current (100A) to flow in the secondary, the power input to the primary being 240W. Calculate: (a) the efficiency of the transformer at full load, unity power factor (b) the load at which maximum efficiency occurs (c) the value of the maximum efficiency.