Three half-lives have passed since the rock are formed.
Amount of time taken by a radioactive material to decay to half of its original value is termed as half life.
Rate law is given by:
t= 2.303/k* log(a/a-x)
where,
k is rate constant
t is age of sample
let initial amount of the reactant be a
amount left after decay process be a - x
a) for completion of half life. we calculate half life as
t1/2= 0.693/k
= 0.693/(1.3 *10^9)
=0.53*10^-9/years
t= 3.9 * 10^9 years
No. of half lives = total time/half time
= 3.9*10^9/1.3*10^9
=3
Therefore ,3 half-lives have passed since the rock formed.
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