skip cell if blank go on to use another cell in calculation
 

I have a worksheet with 2 sets of columns which can be populated with time,
then a couple of columns at the end which contain formulas for total hours.
I use data from set 1 & set 2 for my total hours formula if set 2 is
populated. How do I use data from set 1 for my total hours formula if set 2
is blank? This is the first time I've used this forum, so pls excuse my
inability to convey my need.