AMC12 2011 12A, Problem 5

Last summer $30\%$ of the birds living on Town Lake were geese, $25\%$ were swans, $10\%$ were herons, and $35\%$ were ducks. What percent of the birds that were not swans were geese?

Solution: To simplify the problem, WLOG, let us say that there were a total of $100$ birds. The number of birds that are not swans is $75$. The number of geese is $30$. Therefore the percentage is just $\frac{30}{75} \times 100 = 40 \Rightarrow \fbox{40}$

Answer: 40