I need to estimate the total number of frogs in the pond, let me call it N. This means that the proportion of marked frogs in the observed sample is 10/40, or 1/4. I assume that this proportion is the same as the proportion of marked frogs in the whole population, which is 45/N. Therefore, I can set up a proportion equation: 1/4 = 45/N. To solve for N, I can cross-multiply and get 4 * 45 = N, or N = 180. 
One day, 45 frogs were captured from a pond, marked and then returned to the pond. The next day, 40 frogs were observed in the pond, of which 10 had been marked the previous day. Assuming the marked frogs are equally distributed among all the frogs in the pond, what is the best estimate of how many frogs live in the pond?