For majority of functions the amount of time required for the function to decrease by half depends on the starting point. Type in half-life time and mass2. The formula for a half-life is T12 ln 2 l.
So if there are 1 billion atoms to start with there will on average be 500 million atoms left after a time equal to their half-life.
Since 8 days is 1 half life we just multiply the starting amount by frac 1 2 text30 grams cdot frac 1 2 text 15 grams You can see on Graph 3 that 1 half life is the point 115. If we mess with this a bit we can make it simpler. A certain radioactive substance has a half-life of 12 days. This means that the fossil is 11460 years old.