In general, in one half-life, half of the parent will have decayed.
In two half-lives, half of the remainder will decay, meaning 3/4 in all will have decayed.
Ages on the right (in millions of years) are based on radiometric dating.
A much more detailed subdivision of the timescale has been undertaken in New Zealand, and a local version is used for dating rocks.
When one considers the power of God, one sees that any such conclusions are to some extent tentative. Some fossils are found in Precambrian rocks, but most of them are found in Cambrian and later periods.
We can assume that the Precambrian rocks already existed when life began, and so the ages of the Precambrian rocks are not necessarily related to the question of how long life has existed on earth.
Potassium is present in most geological materials, making potassium-argon dating highly useful if it really works.
Potassium is about 1/40 of the earth's crust, and about 1/10,000 of the potassium is potassium 40.
Assuming we start out with pure parent, as time passes, more and more daughter will be produced. A ratio of infinity (that is, all daughter and no parent) means an age of essentially infinity.
Many dating methods seem to give about the same ages on meteorites.
Thus radiometric dating methods appear to give evidence that the earth and meteorites are old, if one accepts the fact that decay rates have been constant.
We are told that these methods are accurate to a few percent, and that there are many different methods.
We are told that of all the radiometric dates that are measured, only a few percent are anomalous.