One way to determine the density of animals in an area is to use the mark-recapture technique. A number of individuals from a population are captured, marked so they are easily identifiable, and released back into the population. It's assumed that the captured and marked animals will randomly mingle with the rest of the population. Later, the population is re-sampled and the number of marked and unmarked animals is recorded. The following equation can then be used to estimate the size of the entire population. Number of individual marked in first catch/Total population size, N = Number of individual marked in 2nd catch / Total number of 2nd catch

If 25 Prairie Dogs were captured, marked, and released back into their population, and then subsequently 135 Prairie Dogs were captured, of which 18 were marked, what is the approximate total population size, N?