Ill try to be brief

I need a formula that can count the number of terms in a series until the value Im subtracting it from reaches 10 (or less than 10)

I have all the values in the formula below. The idea is to calculate how long itll take stock to reach close to 0 based on the sale of the previous 2 months, factoring in the % increase or drop of sales for that item from one month to the next

n = qty remaining

a = amount sold in last month

x = % change

n - [xa + (x^2)a + x^3)a...] < 10

not sure if im on the right path even, but what I need is the number of terms in the brackets until the formula reaches a value of less than 10