Treat the germination rate as a proportion. The proportion of seeds that germinated would be 240/400 or 0.6.
Since the sample size is relatively large, you can assume that the sample proportion is approximately normally distributed. Using this assumption, you can compute a confidence interval for the true germination rate.
The way to do this is explained at http://stattrek.com/Lesson4/Proportion.aspx . This web page even works out a sample problem that is very similar to the problem you face. Just follow the steps shown in the sample problem, and you can compute a confidence interval for the true germination rate.