The half-life of potassium-40 is 1.3 billion years. An igneous rock is found with 25% potassium-40. What is the absolute age of this rock?

2.6 billion years

Explanation:

Given that :

Percentage of potassium - 40 left = 25%

This is equivalent to 25/100 = 1/4

Half-life(t1/2) of potassium - 40 = 1.3 billion years

Recall; half - life is the time taken for a radioactive substance to Decay to half of its original size ;

Initial percentage of K - 40 = 100%

Time taken to reduce to half = 1.3 billion years ;

After 1.3 billion years, percentage left = 100/2 = 50%

After another 1.3 billion years ; percentage left = 50%/2 = 25%

Hence, time taken to reduce to 25% is (1.3 + 1.3) = 2.6 billion Years

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