Question 906778
Let a, b, c, be the amount of ounces of each of the foods.  Form equations this way:   {{{r[1]*a+r[2]*b+r[3]*c=v}}}, to account for the v quantity of each specified nutrient.  Each {{{r[i]}}} is milligrams of nutrient per ounce of food item.



{{{1*a+2*b+2*c=21}}}, accounting for the milligrams of Niacin.
{{{1*a+1*b+1*c=11}}}, accounts for zinc.
{{{2*a+1*b+2*c=17}}}, accounts for vitamin C.


Solve, any method you want or need.  Matrices be best.