The initial mass of a radioisotope is 10.0 g. If the radioisotope has a half life of 2.75 years, how much remains after four half lives?

After four half-lives, the amount that remains is (1/2)⁴ of the original sample.

That's (1/16) of 10 g, or 0.625 g.

It doesn't matter how long the half-life is.

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