Answer:
2.5 mg
Explanation:
The half-life of a substance is the time required for one half of the substance to decay. In order words, it is the time it will take for a substance to decay into half of its initial size.
For a 20 mg element with a half-life of 30 which decays over a period of 90 years;
The first 30 years, 10 mg would have decayed, remaining 10 mg
The second 30 years, the remaining 10 mg would have decayed to 5 mg
The third 30 years, 5mg would have decayed to 2.5 mg.
Hence, in 90 years, 20 mg of the element would remain only 2.5 mg.
