One way:

B1: =A1 / (1+C1)

This assumes that by "% increase" you mean that C1 has a value of, say,
20% or 0.2. If instead it's 20, use

B1: =A1 / (1 + C1/100)


In article ,
babz wrote:

I need help with a formula. I can't figure out how to calculate for a
missing number.

'05 we sold x units, I know we had a X% increase over '04 so what was
the '04 sales?

Column A = '05
Columb B = '04
Column C = % increase