Spring Cleaning

Exam item 12:

"If the half-life of some radioactive element is one billion years, and a mass of rock originally contained 1000 atoms of the radioactive element, how many atoms of the radioactive element would be left after three billion years had passed?"

... I didn't give half a radioactive sh*t back then, and I certainly don't now. For the record, the answer is 125 atoms.

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Booky goodness for April: A Single Shard, Linda Sue Park.