A scientist conducts a test and finds out that the LD50 of a certain chemical is 20 mg per kilogram. Explain what this means.

The short answer is: The scientist means that a dose of 20 mg per kilogram is enough to kill one half or fity percent of the animals used in the test. LD stands for leathal doseage. The 50 means 50 percent. Subsequently a dose of 20 mg per killogram has a lethal dose that will kill 50% of the test subjects.