13.8 The normal activity of living carbon-containing matter is found to be about 15 decays per minute for every gram of carbon. This activity arises from the small proportion of radioactive ${ }_{6}^{14} \mathrm{C}$ present with the stable carbon isotope ${ }_{6}^{12} \mathrm{C}$. When the organism is dead, its interaction with the atmosphere (which maintains the above equilibrium activity) ceases and its activity begins to drop. From the known half-life (5730 years) of ${ }_{6}^{14} \mathrm{C}$, and the measured activity, the age of the specimen can be approximately estimated. This is the principle of ${ }_{6}^{14} \mathrm{C}$ dating used in archaeology. Suppose a specimen from Mohenjodaro gives an activity of 9 decays per minute per gram of carbon. Estimate the approximate age of the Indus-Valley civilisation.