Time is a 24hour clock. So add up 16 hours and 9 hours and the time result will be 01:00 (25:00 o'clock!). Hence your 20:30 is correct in clock terms. So what I suggested only works up to 24hours - sorry
You will need to convert the result of the sum function to something that looks like hh:mm. This is easy in Excel [h]:mm. However, I don't know how to do this in Access easily. However, you can write a formula to do the trick:
Since the results of the sum is a decimal number of days e.g. 1.5 is 1.5 days, then the above works out the equivalent in hours and minutes and uses some string functions to make it look nice. You could create a function for this if you are planning to use it in lots of reports/forms etc.
It would be interesting to know if anyone knows an easier way of doing this.