Let's first take the equation from a well-known college website to validate my equation as being proper for determining population growth with respect to time:

__http://ftp.columbia.edu/itc/hs/pubhealth/modules/demography/populationRates.html__

From here, you can clearly see that the crude equation for population growth is P(t)=P(0)*e^rt where:

P(t)=P(f) is the final population at the given time (I'll use P(f) going forward to help with understanding that the (f) means the final outcome; what you are left with at the end of said time period.)

P(0)=P(i) is the initial population you start with given the said time period (I changed (0) to (i) to help with the understanding the it's the initial population you start with in the given time period)

e is the mathematical constant for a natural logarithm. (http://en.wikipedia.org/wiki/E_(mathematical_constant))

r is the rate of growth (based primarily on t, this could be seconds, minutes, years, whatever measurement of time you prefer)

t is the time period (sets r as being denoted in seconds, minutes, or years...ie if we were trying to figure out the growth rate from 1950-1980, then we would use years as the unit, and t hence would be 30, making r the annual growth rate)