Adding Time Values together

Hi, I am trying to add monthly working hours together eg 171.25 hours + 56.56 hours but cannot get Excel to do this. When I use HH:MM it changes the cell to a time format but the answer is wrong despite the correct (I think) sum is entered in the total colom ie SUM(B3:CC3). All cells have been set to HH:MM format but again when I enter the hours it changes the value. Can anyone help as I am going mad trying to fix this!?