you need to split your data first then you can calculate percentage... you can use
split_part(string text, delimiter text, field int)
//Split string on delimiter and return the given field (counting from one)
here is example
SELECT (CAST(coalesce(split_part(es.score,'/',2)*100), '0') AS integer)*100)/CAST(coalesce(split_part(es.score,'/',1)*100), '0') AS integer) as 'Percentage' FROM emp_scores es WHERE id = 123;