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