Initially 87 grams of radioactive substance was present. After 4 hours the mass had decreased by 9%. If the rate of decay is proportional to the amount of the substance present at a time . Find the...

Initially 87 grams of radioactive substance was present. After 4 hours the mass had decreased by 9%. If the rate of decay is proportional to the amount of the substance present at a time . Find the amount remaining after 15 hours