Use
=C10/D10
wrapping it in a SUM() function is unnecessary

Use an IF() function to handle zero totals
=IF(D10=0,x,C10/D10)
where for x you would put 0, "", or whatever you like.

Jerry

Virginia Calhoun wrote:

Jerry, I'm sorry I should have explained this better. I am trying to
track the production of each employee and how much production they do in
a day. I have a spreedsheet set up with the first column with their
names, second column with the day, third column is for the units they
did that day, and four column is the hours they worked that day. The
last column is where I want to put the percentage they did that day.
Where there are units and hours worked I put in -sum(C10/D10) and got a
percentage. However, like on Sunday where there is 0 in units and 0 in
hours worked that doesn't work, I get and error. So in those I put
=percentage(C10:D10,0.3) and it worked. But lets say I forgot a couple
of employees worked that day and I went back and put in the units they
did and the hours they worked instead of the 0, then it gave me error
not a percent. Plus it seems to me I should have the same formula all
the way down the column so this doesn't happen, correct? There are no
definent amount of units they should do in a day. So I am unclear how
to do this. Any suggestions? Thanks Jen