SQL code unable to display attendance in percentage format

SQL code unable to display attendance in percentage format

I am having trouble adding the condition to only show those with
attendance less than 81..
Select Absence.StudentID as StudentNumber,
Student.Name as "Student Name",
Subject.Name as "Subject Name",
CONVERT (varchar, 100-(100 * count(*) /10)) as 'Attendance(%)'
FROM Absence,Subject,Student,
(SELECT COUNT(*) as tot FROM Absence) x
WHERE Subject.SubjectCode=Absence.SubjectCode AND
Student.StudentNumber=Absence.StudentID
GROUP BY Absence.StudentID,Subject.Name,Student.Name;
The above is the code that displays exactly what I want but I cannot add
in the condition only. My Attendance(%) is already int right? So using the
CAST or CONVERT does not work either. It says it has trouble converting.
Your kind assistance will be greatly appreciated.