Answer: The rock is 2000785 years old.
Explanation:
Expression for rate law for first order kinetics is given by:

where,
k = rate constant
t = age of sample
a = initial amount of the reactant = 800
x = amount of daughter isotope = 600
a - x = amount left after decay process = amount of parent isotope = (800-600) = 200
Half life is the amount of time taken by a radioactive material to decay to half of its original value.




The rock is 2000785 years old.
The cycles within which earth's matter moves are closed systems. This means nothing is gained or lost from the system; it only changes from one form to the other. The total amount of matter on earth remains the same. Using tectonic plates as an example, divergent boundaries occur where new oceanic crust is being created. New crust is being created and simultaneously the same amount of crust is being destroyed at convergent margins.