Answer:
If 0.25 oz of yeast is used, then 50oz of flour is needed.
It would require 0.5 oz of yeast for 90 oz the flour.
Step-by-step explanation:
We know that the proportion of yeast (y) and flour (f) is 1: 200 when a maximum of 30 servings of bread is made. This is:

So if we use 0.25 oz of yeast:
if 0.25 oz of yeast is used, then 50oz of flour is needed.
If you make 60 servings of bread with 90 oz. Of flour, then you should keep the following ratio:

So, if we use f = 90 0z of flour:
It would require 0.5 oz of yeast.