Determining Weeks of Supply (WOS)

Help!

I am having trouble determining how many weeks of inventory I have on-hand. See example data below.

Sales Projections

Wk 1 = 50
Wk 2 = 50
Wk 3 = 15
Wk 4 = 15
Wk 5 = 25
Wk 6 = 26
Wk 7 = 18
Wk 8 = 20

Inventory on hand = 115

I know the answer is I have 3.0 wos because 50 + 50 + 15 = 115, but what's the formula to get excel to spit out the answer? It's almost like I need a formula to first identify the fall off point and then return that value to another formula that would then do some math based on that returned range.

Thanks for the help in advance!