Naturally occurring zirconium exists as five stable isotopes: 90Zr with a mass of 89.905 amu (51.45%); 91Zr with a mass of 90.906 amu (11.22%); 92Zr with a mass of 91.905 amu (17.15%); 94Zr with a mass of 93.906 amu (17.38%), and 96Zr with a mass of 95.908 amu (2.80%). Calculate the average mass of zirconium.