Since we want to run our system at 30MIPS, we want 30MHz out of our PLL. Since the VCO must output a frequency between 100 and 200MHz, we have to find a frequency that the VCO can run at which is a multiple of our 30MHz. The post-divisor can be either 4, 8 or 16 and if we choose 4, this will allow the VCO to run at 120MHz. This is within our range and 120 divided by 4 is 30MHz.