his is a step down transformer since n(primary) is greater than n(seconcary). You relate the input voltage with the ouput voltage with the following equation:
Vout = n2/n1*Vin (n2/n1 is essentially your 'transfer function' that dictates what a specified input would produce)
Solving the equation:
Vin = Vout*n1/n2 = (320V)*(600/300) = 640 V
This is checked by seeing if Vin is greater than Vout, which it is for a step down transformer.