Advice on Equations

Hi - need some help on formulas.

I have two objects:

a. produce amount - which has a date and an amount (volume)

b. area - which is are sets of farm units (area)

What I am trying to figure out is how would I calculate:

"amount per area unit per year"

I can figure out using equations to calculate totals/etc but how would I dynamically create the equation to work out amounts per year.

I think I am too tired to see the light on this one!