No real problem. Your math is probably fine; formatting needs fixing.

Select the cell with the sum in it.
Format this cell as Time 37:30:55
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Gary's Student


"Michelle" wrote:

I am summing up hours in Excel and the sum formula is not working Properly.
For 2 of my 5 cells are adding correctly, but the other 2 when added to the
formula throw the entire thing off.

They are all formatted the same in 13:33 format to measure the # of hours
spent on an activity.

What would the reason be that two of them are not working? (it is almost
like exel is substracting hours when these 2 cells are added)