Formula To Get An Average Based On A Dropdown List

Im creating a spreadsheet at work, which has a drop down box with the options of open and closed in it.

I also have a column for length of time of the claim, what i need to do, is now find a way of getting a average length of a claim, but only to see the closed option from the drop down box.

for example,

row 1
open date - 02/06, closed date - 01/07, dropdown box - closed, life - 29
row 2
open date - 10/06, closed date - n/a, dropdown box - open, life - -39400

as you can see from the example above, i cant simply do a average of the whole column, as would read the open ones as well. so is there a way of doing the formula, but telling it to only see the open, or closed ones?