An exponential function has the form y=abx, where a=0 and the base b is a positive real number other than 1. Let's determine and interpret a and b of the given model. y=2500(1.50)t In this situation, a represents the number of subscribers it received at the beginning of the model. Because b is greater than 0 and less than 1, it represents the growth factor of the number of referrals each year. To determine the annual percent increase, we should consider the exponential growth model. y=a(1+r)t In the model, r is the percent increase written as a decimal and the quantity 1+r is the growth factor. Knowing that the growth factor is 1.50 in our situation, we can find the percent increase. 1−r=1.50 ⇒ r=0.50 or r=50%