A radioactive isotope, Phlebotinum, decays into Dilithium with a half-life of 42 million years. If we find a rock with 25% Phlebotinum to 75% Dilithium, how old is the rock? How do we know?

time taken for 25% Phlebotinum and 75% Dilithium is 168 million years

Explanation:

radioactive isotope of Phlebotinum decays into Dilithium.

time taken to half life takes = 42 million years

half life means that the Phlebotinum is 50% and Dilithium is also 50%

and now it is given that Phlebotinum is 25% and Dilithium is 75%

hence this condition will come after two half lives.

for one half life it takes 42 million years

and for second half life time becomes 4 times of first half life

time for 25% Phlebotinum and 75% Dilithium is = 4 * 42

= 168 million years.

hence time taken for 25% Phlebotinum and 75% Dilithium is 168 million years