1. A radioactive substance with an initial mass of 250mg has a half-life of 1 year. a) Write an equation to relate the mass of radioactive material remaining to time. (4 marks) Y = a ( 1 - decay rate ) DS Y = 250( 10 12 b) What mass will remain after 10 years? (4 marks) wors the question shown? b) How long will it take for the sample to decay to 20% of its initial mass? Explain how you arrived at your answer. (4 marks)