I know how to do the sum of cells:

In cell -- c18, I have the formual "=average(c3:c17)"

My question is, if there is not value entered in one of those cells, how do
I get it to not add it in? I want C18 to omit the cells (as part of the
average) if it is blank, and only add in the average if something is entered.
(SEE EXAMPLE BELOW) I would want the average of only c6 & c7 (which in this
case is 5), since they have a number entered. I only want cell c8 to be
included if there is a number. In other words, I do not want the average to
drop if there is nothing entered.

For Example:

c6=5
c7=5
c8=(nothing entered)

Any help would be much appreciated. Thank you.