The amount of substance left of a radioactive element of half life,

after a time, t, is given by:

Given that <span>potassium-40 has a half life of approximately 1.25 billion
years.
The number of years it will take for 0.1% of potassium-40 to remain is obtained as follows:

Therefore, </span><span>the maximum age of a fossil that we could date using 40k is
12.5 billion years.</span>
To do this, you can just multiply across. 3 times 1 equals 3 so your numerator is 3. 2 times 4 equals 8. Then, your answer would be 3/8. Hope this helps :)