HELP with Throughput Accounting BPP Revision kit question 15

The answer says that reducing the machine time will have no effect on TA ratio because the factory cost per hour and throughput per hour will increase by the same amount so the TA ratio will remain unchanged. But I don’t quite understand how the machine time per unit affects the factory cost per hour. (I have factory costs as the fixed costs; and cost per factory hr = FC/Total time available; TA ratio =RPFHr/CPFHr). So I don’t see how machine time per unit affects the factory cost per hour) Is it possible for you to illustrate this by an example. i went over the lecture twice but cant seem to connect it with this.