Answer:
$2.40
Explanation:
Here is the complete question: A deli sells 320 sandwiches per day at a price of $4 each.
A market survey shows that for every $ 0.10 reduction in the price, 40 more sandwiches will be sold. How much should the deli charge in order to maximize revenue?
Given: Number of sandwiches sold per day is 320.
Price of each sandwich is $4.
For every $0.10 reduction in the price, 40 more sandwiches will be sold.
Let´s assume price of each sandwich sold at new reduced price be "x" times.
∴ New price of sandwich is 
As given, for every $ 0.10 reduction in the price, 40 more sandwiches will be sold.
∴ Total number of sandwiches sold at new price= 
We know, revenue= 
⇒ revenue= 
using distributive property of multiplication.
⇒ revenue= 
Considering the revenue to be R(x)

Now taking derivatives of the revenue.
= 
Solving it.
We get, 
Finding critical points by setting derivative equal to 0
R´(x)⇒ 
Subtracting both side by 128
⇒ 
Dividing both side by -8
⇒ 
∴ x= 16
Hence, price of each sandwich is reduced by $0.10 is 16 times.
Next, finding cost of each sandwich to maximize revenue.
cost of each sandwich to maximize revenue= 
∴ Cost of each sandwich to maximize revenue= 
Hence, Deli should charges $2.40 for each sandwich to maximize revenue.