I want to create a spreadsheet in which I record total number of hours spend
on a maintenance job whereby minutes less then 30 are rounded to half an hour
and more then 30 are rounded to the hour.

Eg : 5:30 should be displayed as 5.5; 2:15 should become 2.5. 4:45 should be
come 5.0.
Which function should I use to facilate entering the data?

Next I want to sum the total number of hours and minutes. Obviously, if the
sum of minutes if greater then 60 the number of hours should be increased.
Example : 15 hours and 90 minutes should become 16.5