The radioactive decay obeys first order kinetics
the rate law expression for radioactive decay is
![ln\frac{[A_{0}]}{[A_{t}]}=kt](https://tex.z-dn.net/?f=ln%5Cfrac%7B%5BA_%7B0%7D%5D%7D%7B%5BA_%7Bt%7D%5D%7D%3Dkt)
Where
A0 = initial concentration
At = concentration after time "t"
t = time
k = rate constant
For first order reaction the relation between rate constant and half life is:

Let us calculate k
k = 0.693 / 72 = 0.009625 years⁻¹
Given
At = 0.25 A0

time = 144 years
So after 144 years the sample contains 25% parent isotope and 75% daughter isotopes**
Simply two half lives
Answer:
The lithosphere And the upper mantle combines the two crusts, oceanic and continental, with the coolest part of the Crust?. The asthenosphere allows the tectonic plates to slide on the putty substance of the mantle and the mesosphere is the Smallest? part of the mantle. The outer core is hot enough to turn the metal into a Liquid The inner core is the hottest but the extreme pressure makes it a Solid.
Physical change .. you cannot undo chemical ... physical is like bending , shattering a mirror , denting something ..
chemical would be rust or something .