An unknown radioactive element decays into non-radioactive substances.

In 320\displaystyle {320} days the radioactivity of a sample decreases by 76\displaystyle {76} percent.

(a) What is the half-life of the element?

  (days)

(b) How long will it take for a sample of 100\displaystyle {100} mg to decay to 88\displaystyle {88} mg?

  (days)

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