Sergei uses 4 cups of ice cream to make 3 milk shakes.
Number of cups needed to make 1 milkshake is calculated as,
[tex]No.\text{ of cups }\frac{4}{3}[/tex]Number of cups needed to make 5 milk shakes is calculated as,
[tex]5\times\frac{4}{3}\text{ = }\frac{20}{3}\text{ = 6}\frac{2}{3}[/tex]Thus the number of cups required to make 5 milkshakes is
[tex]6\frac{2}{3}[/tex]