Update Available Seats After Enrollment

[pallav_nagar]

How to Update Available Seats After Enrollment in Employee Enrollments record using Business Rules in ServiceNow?

Table - Employee Enrollments

Scenario:
Whenever an employee enrolls in a course, automatically decrease the Available Seats count in the related Training Course.

Additional Field(s) Required:
Available Seats (Integer), Total Seats(Integer) on the Training Courses table.

Expected Result:
Each new enrollment automatically reduces the number of available seats.

[Mark Manders]

What did you already try and what didn't work? Especially with a BR, you can just do calculations on the field (enrollment added, look up training course and deduct 1 from the available seats).