Question 1191133
Their profit is $4 per sale currently
for each $1 increase they lose 2 sales.
profit is 4+x, and sales are 20-2x
so maximize that profit which is f(x)= 80+12x-2x^2
x=-b/2a maximum or -12/-4 or 3, and f(3)=80+36-18=$98
so when x=3 the sale price is $13 (profit is $7) and the sales number is 14 (20-3*2)
The profit is maximum $98 at $13 per decoration sale price. 

Check
#---price-#---revenue-- N-CP(number-cost to produce)---Profit
6---$10---20--$200------$120-----------------------------$80
6---$11---18---$198----- $108-----------------------------$92
6---$12---16--$192------$96------------------------------$96
6---$13---14--$182------$84------------------------------$98
6---$14---12--$168------$72------------------------------$96