I have to determine the average atomic mass of the following mixtures of isotopes.

Please help me understand and guide me. I have to determine the average atomic mass of the following mixtures of isotopes.

1) 80% 127I, 17% 126I, 3% 128I

2) 50% 197Au, 50% 198Au

3) 15% 55Fe, 85% 56Fe

4) 99% 1H, 0.8% 2H, 0.2% 3H

5) 95% 14N, 3% 15N, 2% 16N

6) 98% 12C, 2% 14C