PRACTICE / Mean life of a radioactive population

Two different decay timescales

A parent isotope obeys exponential decay with half-life 6.0 hours.

Find its mean life.

1. A$\frac6{\ln2}\ \mathrm h$
2. B$6\ \mathrm h$
3. C$6\ln2\ \mathrm h$
4. D$12\ \mathrm h$

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