how to subtract two date columns & calculate the duration in MSSQL

I have tree columns in one of the tables: check-in time and check-out time, billableHours. check-in & check-out are date fields. billableHours is varchar and would like to format like this: HH:MM.

I could use following query

SELECT CONVERT(VARCHAR,(Checkout-checkIn),108) from WorkTrackingLog

but I get HH:MM:SS

Question # 1: How do I subtract checkout-checkin and get HH:MM?

Now once I get HH:Mm from check-in & check-out date, I would like to add these numbers and get the totalDuration in other table.

For example lets say I have three records:

Table # 1:

------------------------

TicketId |  billableHours

------------------------

1001    |           05:04

1001    |           12:19

1001    |           02:16

------------------------

Table # 2

------------------------

TicketId | totalDuration

------------------------

1001    |           19:39

------------------------

Questions # 2 - How do I convert HH:Mm to int and add multiple records together to get totalDuration. Total duration can be varchar format.

Make sense?

thank you in advance for your input/feedback.