Answer: (d)
Step-by-step explanation:
Given
Annual profit is given by
[tex]P=10x^2-280x+1600[/tex]
When it is factored then,
[tex]\Rightarrow 10x^2-280x+1600=0\\\Rightarrow 10\left( x^2-28x+160\right)=0\\\Rightarrow x^2-20x-8x+160=0\\\Rightarrow x(x-20)-8(x-20)=0\\\Rightarrow (x-8)(x-20)=0\\\Rightarrow x=8,20[/tex]
On factorization, we get that annual profit is zero at a whole prize of $8 and $20.