Summing Values In Column Based On Values In Other Column

I have a vba form with three comboboxes. The first combobox contains a list of employees, the 2nd a list of work activities and the third a list containing hours. As each employee enters the data for their name, activity and hours these values are saved to sheet6 starting at A10, B10 and C10. I want to be able to sum the hours for each employee who enters data. So the completed user form could look like the following:

HTML
Employee (A) Activity (B) Hours (C)
Joe Swept 1.25
Raked 1
Bill Mowed 2
Cleaned 1
Joe Trimmed 1

So as you can see there could be a blank cell in column
"A" under Joe and Bill and Joe could come back later and
enter more info. What I want on sheet6 say starting at A30 the value
Joe would be entered and in B30 his total hours 3.25. In A31
Bill would be entered and in B31 his total hours 3 and so on for other
employees