For this case, the first thing we must do is define a variable.
We have then:
x: number of bushels of grain each week
y: total cost
So, for supplier A we have:

Evaluating for x = 500 we have:


For supplier B we have:

Evaluating for x = 500 we have:


We note that the cost of supplier B is higher.
Then, the difference in the cost of both suppliers is:

Answer:
Supplier B would cost $ 625 more than A