Well, a half-life is the amount of time it takes for a radioactive substance to decay by half (mass decays into another, more stable substance in series). So if 1/10 (or 0.1) of the half-life of 1.3 billion years has passed, that's 1.3 X 0.1 = .13 billion years, or 130 million years since the ash was deposited.
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