1. lead has an average atomic mass of 207.19 amu. the three major isotopes of lead are pb-206 (205.98 amu); pb-207 (206.98 amu); and pb-208 (207.98 amu). if the isotopes of pb-207 and pb-208 are present in equal amounts, calculate the percent abundance of pb-206, pb-207, pb-208.