A certain type of radioactive isotope decays over time. The mass of the isotope, in grams, can be modeled by an exponential function $M(t)=M_0{a}^t$, where $t$ is the number of years. If the mass was 80 grams after 2 years and 72 grams after 3 years, by what percentage does the mass of the isotope decrease each year? (Round your answer to the nearest tenth of a percent, if necessary. Do not include the percent sign.)