Create Sum based on IF statement formula?

Hello!

I am seeking some help/advice/magic with a function on a spreadsheet I am creating. I am looking to create an average of a sum of numbers [O52:53], but wish to only calculate information from one column [either column U or V] for each row, disregarding one of the columns in the SUM formula. This is due to having two different courses along the row [either K through R or L through R] that creates an average amount of days from two different potential start dates. I don't want to skew the total number of days because it is adding them both together.

So, is there a function to have data not appear contingent on information in another cell? Like, if the information were to flow on Row 9 and thus calculate the needed total in U9, can I make it so that there is no value calculated in V9? Could this be contingent on a "yes" or "no" answer in column D?

As can be seen in the attached sheet, this is also an issue in the other possible option of needing data from L through R to be calculated. Can it be done so that there is no value represented in column U in this case? These formulas would have to work at the same time as it won't be predicted as to how each row will be completed (either starting with K or L for the beginning date).

Any help would be GREATLY appreciated!!!