The decay of a radioactive substance can be modelled by the equation ?? = ??0?? -????, where ??, measured in grams is the mass of the substance after ?? years. After 5 years the mass of the substance...

The decay of a radioactive substance can be modelled by the equation ?? = ??0?? -????, where ??, measured in grams is the mass of the substance after ?? years. After 5 years the mass of the substance is 196 grams and after 10 years the mass is 49 grams.