The total revenue for billing purposes is currently $425,000 x 15% = $63,750. We want to get a unit of revenue per unit time and this is where the 75% comes in.
Dividing by 75% is the same as finding the "revenue per unit time" figure since the 75% corresponds to the actual amount of time that the physician earns money: remember that he doesn't earn money on administrative and billing work but only on face-to-face time.
So to get a standard measure of "revenue per unit time" we need to divide by 75% since we have now taken the other two roles away which are freed up by the outsourcing.
Recall that with the other roles patient time is 75% and without these patient time is 100%. The ratio of these is 100%/75% = 1/75%.
This 1/75% is a conversion factor of going from one distribution of activities to another.