Answer :
To solve this question we need to know the concept behind half life. Decay of radioactive element always comes under first order kinetics. Three half-lives have passed since the rock formed.
What is half life?
Half life tells about the time at which the radioactive material decays to half of its initial concentration.
Mathematically the the total number of half life can be calculated as
[tex]\rm \frac{t}{t_{1/2}} =-\frac{ln\frac{N_{t}}{N_{0}} }{ln 2}[/tex]
Nt= amount of the isotope that has not yet decayed after a time t
N₀= initial amount of the isotope
t=the time
[tex]t_{1/2}[/tex] = the half-life
Substituting the given values in the above equation
[tex]\rm \frac{t}{t_{1/2}} =-\frac{ln\frac{125}{100} }{ln 2}[/tex]
Solving this we get
3[tex]t_[1/2}[/tex]= t
Therefore, three half-lives have passed since the rock formed.
To know more about half life, here:
brainly.com/question/21464063
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