First solve for the lower half of the diagram.

Since the entire diagram is representative of $100\%$ of the students, and the first portion represents $60\%$, the second portion must represent $40\%$.

$28$ students is $40\%$ of how many total students?

To find the remaining amount of students for the first portion, subtract $28$ from the total amount of students.

$\frac{40}{100}\left(\frac{?}{?}\right)=\frac{28}{x}$