7. a) A certain radio-active isotope has a half-life of 6 months. Starting with a sample of 1500 g of radio-active material, state an algebraic model that should be used to determine the amount of material remaining after t months? Include what the variable(s) represented. [3] b) How much of the isotope will remain after 10 years?[2] c) How long will it take for the isotope to have only 265g remaining?[2]