How do you make an equation for this?

it means you take 1 mL of inoculum (from your first tube) and add it to 9 mL of diluent. this is a 1 to 10 dilution that is represented as 1 x 10^-1. you the take this dilution and do the same thing. this gives you a 1 x 10^-2. you need to do this seven more times to get 1 x 10^-9. JuJu:Revitalized describes the scientific notation the best.