Percentiles and Box Plots · Challenge problem

A student's percentile rank in a group is the percent of the group (including the student) who scored strictly lower. Sam is in class A, where $28$ of the $40$ students scored lower than Sam. Sam's score is also higher than exactly $80\%$ of the students in class B. When the two classes are combined, Sam's percentile rank is $76$. How many students are in class B?