Answer:
restaurant should charge $(6-0.25) = $5.75per sandwich to maximize daily revenue.
the revenue is $1983.75
Explanation:
to calculate current revenue= $6 x 330 = $1980
suppose x as the number of times the price to be dropped by $0.25
 then find new price.. i.e 
new price= $(6-0.25x)
and, new sell=330 +15x sandwiches
therefore, the new revenue would be= (6-0.25x)(330 +15x)
in order to maximize the current revenue, simplify the above equation and make it complete square using x
(6-0.25x)(330 +15x)
=1980-82.5x +90x -3.75

=1980 + 7.5x -3.75

=1980-3.75 (-2x+
 ) ----> taking out common
) ----> taking out common
now, to make a complete square lets add and subtract 1 inside the parentheses
=1980-3.75(-1+1-2x+
 )
)
=1980 +3.75 -3.75(
 -2x +1)
 -2x +1)
=1983.75 -3.75 
 ---->(1)
---->(1)
as 
 is positive always, minimize the other term in order to maximize the total revenue.
 is positive always, minimize the other term in order to maximize the total revenue.
so the minimum possible value of 
 = 0
 = 0
therefore, x=1
putting x in eq(1) the revenure becomes,
$(1983.75-0)=> $1983.75
therefore, restaurant should charge $(6-0.25) = $5.75per sandwich to maximize daily revenue.
the revenue is $1983.75