Can anyone help? I can't think of a way to do this in a single cell
expression.

I have a column of week ending dates next to weekly sales next to end
of week stock holding and I need to add a column that shows how mnay
weeks the stock will last for providing I buy no more stock. Rough
example below.

Week Sales Stock Cover
1 150 500 3.2
2 150 500 2.8
3 150 700 3.6
4 150 800 5.0
5 250 800 5.?
6 200 700 ?
7 150 600 ?
8 100 500 ?
9 100 500 ?
10 100 500 ?

I hope that helps. It need to take forward sales off until it get to a
part week and then work out the fractions like. 500-(150-150-150)/250
= 3.2 weeks cover. I hope this is clear... I just need a while
statement but alas Excel does have that and I can't figure a complex
way to use If conditions...