The function a=a_oe^{-0.01155x} a = a o e − 0.01155 x models the amount in pounds of a particular radioactive material stored in a concrete vault, where a_o a o is the original amount of material and x is the number of years since the material was put in the vault. if 900 pounds of material are placed in the vault, how much time will need to pass for only 357 pounds to remain? round your answer to the nearest tenth.