6.) a deli sells 640 sandwiches per day at a price of $8.00 each. a market survey shows that for every $0.10 reduction in price, 40 more sandwiches will be sold. how much should the deli charges for a sandwiches in order to maximize revenue?

The demand function is

... q(p) = 640 + 40(8.00-p)/0.10 . . . a drop in price of 0.10 sells 40 more sandwiches

... q(p) = 3840 -400p . . . . . . . . . expanded form

... q(p) = 400(9.60 - p) . . . . . . . . factored form

The revenue is the product of price and the quantity sold.

... R(p) = p·q(p)

... R(p) = 400p(9.60 - p)

This is the equation of a quadratic function with zeros at p=0 and p=9.60. Since the zeros of a parabola are symmetric about its vertex, the vertex (maximum) will be halfway between those values, at p=4.80.

The deli should charge $4.80 for a sandwich to maximize revenue.

1/8 is left? add a bit more info plz