According to the problem, a 1/3 cup of beans is equivalent to 4 servings. To know the number of cups of beans for 28 servings, we just have to use the rule of three.
[tex]x=28\cdot\frac{\frac{1}{3}}{4}=28\cdot\frac{1}{12}=\frac{7}{3}=2\frac{1}{3}[/tex]