The half - life of barium-137m is 2.552 minutes. how much of an 18 gram sample would be left after 10 minutes

To simplify the problem imagine the half-life was 2.5 minutes. This means that there would be 10/2.5=4 periods of halving the mass, which means 1/2⁴=1/16 of 18g=1.125g.
So for the given half-life we calculate 10/2.552=3.9185 approx. So we now find 2 to this power=15.1211 approx and divide into 18=1.19g. This is close to the rough estimate we did earlier.

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