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Answer:
Age of rock = 21112000000 years
Explanation:
The half-life of a radioactive material is the time taken for half the original material to decay or it is the time required for a quantity to reduce to half of its initial value.
The ratio of parent isotope, Uranium-235 to daughter isotope, Lead-207 = 6 : 42 = 1 : 7
This means that for every one gram of the parent isotope, there are 7 grams of the daughter isotope. So,the quantity of radioactive material left is one out of eight its original value.
Number of half-lives undergone for 1/8 of the original value to to remain is given below:
1/8 = 1/2 × 1/2 × 1/2
Therefore, number of half-lives = 3 half-lives
Age of rock = half-life of Uranium -235 × number of half-lives
The half-life of Uranium-235 is 704 million years = 704000000
Age of rock = 704000000 × 3
Age of rock = 21112000000 years