As lurker pointed out in his comment, you will need to use the is/2
standard built-in predicate to make the calculations. Something like:
got_perc(Person, Perc) :-
% get student work and exam grades
marks(Person, Work, Exam),
% calculate the student final grade
Perc is Work*0.25 + Exam*0.75.
The is/2
standard built-in predicate, which is also defined as an infix operator, unifies the left operand with the result of evaluating the right operand as an arithmetic expression.