Excel 2003

Hello, I am trying to calculate weeks on hand.

I have a forecast for an item by week (A=week1, B=week2, etc) I also have
the total quantity on hand. I need a formula that will calculate how many
cells (weeks) = the total quantity on hand. Example:
A1=99; B1=105; C1=89; D1=102 (forecasted quantities)
A2=312 (quantity on hand)
A3=3.2 (weeks on hand- rounded to tenths)- this is where I need the
formula.

Thanks in advance for any help given.