Sum Time columns in SQL

I'm writing a query to calculate when the students check-in and check-out and how much time they spend at the GYM. I have a Checking Field that contains the check-in time and a check-out field that contains the check-out, and a Duration.

Check In        Check Out      Duration
----------------------------------------
12:30:00        1:30:00         1:00:00

How can I calculate check-in and check-out to obtain a duration like in the example?

#sql #t-sql #sql-server