5. the half-life of a certain radioactive material is 78 hours. an initial amount of the material has a mass of 790 kg. write an exponential function that models the decay of this material. find how much radioactive material remains after 18 hours. round your answer to the nearest thousandth.,

Answer asap and if you do you will get brainliest. catherine buys a gallon of ice cream from the store. after taking it home, she eats a fifth of a gallon of ice cream. her sister eats some of the ice cream as well. if two-thirds of the original amount of ice cream is left, then what fraction of a gallon of ice cream did her sister eat?