Would it be simpler to use the built-in time functions? If e24 and f24 are already in Excel's time format, then you can simply have the sum be in a time format and it'll convert the display to 22:10 automatically for you.
To convert the result manually, do something like the following:
Say:
g24=sum(e24+(f24/60))
h24=int(sum(e24+(f24/60))) - gives the hours
i24=int(60 *(g24-h24)) - gives minutes
I think that'll work. I don't know off the top of my head how you'd display them as 22:10 though.
HTH a little.
Cheers,
Scott.