Asubstance decays so that the amount a of the substance left after t years is given by: a = a 0 · (0.9) t , where a 0 is the original amount of the substance. what is the half-life (the amount of time that it takes to decay to half the original amount) of this substance rounded to the nearest tenth of a year? 5.6 years 6.1 years 6.6 years 7.5 years