hello. i am not a mathematician. i'm just a guy who's been up for too many hours today...

i want to sell a cumulative total of X widgets over Y number of weeks. i know that my first week’s sales will be Z. i want to increase my sales at the same rate each week from Z in the first week through to the last week (and ending at my cumulative goal of X).

how do I calculate the rate of increase? can i?

p.s., i'm not a salesman either. just could not think of another way to explain what i'm trying to do...