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Answer:
(1/2)*n = .05 by the definition of half-life where n is the number of half-lives
n ln .5 = ln .05
n = ln .05 / ln .5 = 4.32 number of half-lives
So 4.32 * 20 = 86.4 years has passed
Actually, they only want the mass left. However, all that has changed is that
95% of the original radioactive atoms have changed to a different form of about the same amount of mass. The amount of mass remaining would be about the same. Also, one doesn't know the percentage of radioactive atoms
that formed the original mass.