This utilizes the SIR Model. S flows into I and I flows back into S, there is no R as this is a modified version of the model. SIR Model is typically used to predict disease transmission, however it is now be used to calculate subscription patterns. S' = -aSI +bI, I' = aSI - bI; b = 1/160 and a = rate of transmission. 20 people joined after t = 1, "a" must be deduced and put back into the model.