A certain element x has four isotopes. 5.845% of x has a mass of 53.93961 amu. 91.75% of x has a mass of 55.93494 amu. 2.123% of x has a mass of 56.93539 amu. 0.2820% of x has a mass of 57.93328 amu. what is the average atomic mass of element x?