Isotopes and average atomic mass

A sample of oxygen atoms contain only ._(8)O^(16) and ._(8)O^(18) isotopes. If the average atomic mass of the sample is 16.8, then identify the options which is/are correct ?

The element bron occurs in nature as two isotopes havein atomic masses 10 u and 11 u . What are the percentage abundances of these isotopes in a sample of boron having average atomic mass of 10.8 u?