Answer:
B. 3
Explanation:
The half-life of a radioisotope is the time it needs for the mass of the radioisotope to halve with respect to its original value.
In this problem, the initial mass of the radioisotope at t=0 is
m0 = 50.0 mg
We see that after t = 1 min, the mass of the isotope is
m(1 min) = 25.0 mg
so, exactly half the initial mass: this means that 1 minute is exactly the half-life of this radioisotope.
So, the amount of mass left after each minute is the following:
m (1 min ) = 25.0 mg (1 half-life)
m (2 min) = 12.5 mg (2 half-lives)
m (3 min) = 6.25 mg (3 half-lives)
so, when we are left with 6.25 mg of isotope, 3 minutes have passed, which means that 3 half-lives have passed.
