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

8

explanation:

4+4=8

the first phase of the government’s plan for settlement was building the transcontinental railroad. the railroad provided a way to bring settlers and manufactured goods west and ship their agricultural and mining produce east. the transcontinental railroad was an essential artery for rapid development of the frontier.

the second phase of the government’s plan was a liberal land distribution policy that made it possible for many people to homestead. with these two key elements—transportation and cheap land—the government rapidly achieved its goal of persuading people to move west, settle on farms, and push back the frontier.