Average+Atomic+Mass

====Naturally occurring boron is 80.20% boron-11 (atomic mass-11.01amu) and 19.80% of some other istopic form of boron. What must the atomic mass of this second isotope be in order for the 10.81amu average atomic mass of boron?====

====What is the atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176, 19 have a mass of 177, 27 have a mass of 178, 14 have a mass of 179, and 35 have a mass of 180.0? ====

====Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following atomic masses and abundances in nature: 6.017 amu, 7.30% and 7.018 amu, 92.70%. ====