Total weight required is half pound, so let the amount of cashews be [tex] a[/tex] , and amount of peanuts be $b$
$\therefore a+b=0.5$ (1)
And we want this to cost $2.80$
Cost of $a$ pound of cashews will be $5.50\times a$ and cost of $b$ pound peanuts will be $2.30\times b$
$\therefore 5.5a+2.3b=2.8$ (2)
Substitute $a=0.5-b$ from equation (1) in equation (2)
$5.5(0.5-b)+2.3b=2.8$
$\implies -3.2b=2.8+5.5\times0.5 $
$\implies -3.2b=0.05$
$b$ comes out to be negative. That means there's no solution with the given conditions.
I checked once , I don't think there's any mistake. Can someone else verify too?
EDIT
I verified all the given options from calculator, and no option gives 2.80