How old is a rock if it contains a radioactive element with a half life of 100 million years?

There isn't enough information in the question in order to provide a definitive answer. We would need to know how much of the rock contains the radioactive element and/or how much of the stable "daughter atoms" are present from the decaying radioactive "parent atoms." Knowing the half-life gives you the rate of decay, but without knowing how much radioactive decay has already happened, determining the age of the rock isn't realistically possible. A half-life is an interesting thing, because no matter how much decay has occurred, there will always be some radioactivity present. This is because the half-life determines how long it will take for half of the radioactivity to decay, and it's possible to continue dividing by half forever, because you just use smaller numbers.
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