Answer: 2 grams of the initial mass remains after 90 years.
Explanation:
The initial amount of the radioactive element is 16 grams. The half-life of the radioactive element is 30 years.
The mass y(in grams) remaining after t years is given by
y=16(1/2)^t/30 ,t ≥ 0
Therefore, the amount of the initial mass that remains after 90 years would be
y=16(1/2)^90/30
y=16(1/2)^3
y = 16 × 0.125 = 2 grams