Hello!
The typical amount of arsenic in a 60 kg adult would be
15,6 milligrams. To calculate that number, we take the data that the fraction of mass of arsenic in the human body is 260*10⁻⁹ and we apply the following conversion factor.

Arsenic is a dangerous element for health, but in very small amounts is one of the
essential elements of life. It is incorporated into the body through the eating of meat and especially fish and crustaceans.
Have a nice day!