Answer:
3.9 billion years
Explanation:
The half-life of potassium-40 is the time it takes for half of it to decay.  
After one half-life, half of the original amount will remain.  
After a second half-life, half of that amount will remain, and so on.  
We can construct a table as follows:  

We see that the mass will drop to 10 g after three half-lives.
   1 half-life = 1.3 billion years
3 half-lives = 3.9 billion years
 
        
                    
             
        
        
        
That answer is true   hope that helps you 
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Answer:
The drinking water in a location in Florida had a fall order and test and this happened due to the storm water runoff that had carried huge amounts of soil and minerals into it. 
The solution for this problem is that the people of that local area should practice afforestation.
Explanation:
Afforestation is defined as a process of planting more and more trees in an area. The plants are helpful for the people and environment in many ways. The most important thing is that the plant reduces the soil erosion in an area. 
The root of the plant and small hub tends to hold up the soil and prevent them to flow away from the high speed runoff water into the water body. 
Also it provides the favorable conditions for more animals and birds. It also provide fresh air to the local people to inhale and utilizes all the carbon dioxide.
 
        
             
        
        
        
Explanation:
Survival of the fittest is a phrase that originated from Darwinian evolutionary theory as a way of describing the mechanism of natural selection.In On the Origin of Species, he introduced the phrase in the fifth edition published in 1869, intending it to mean "better designed for an immediate, local environment".