(a) Two stable isotopes of lithium `""_(3)^(6)Li` and `""_(3)^(7)Li` have respective abundances of 7.5% and 92.5%. These isotopes have masses 6.01512 u and 7.01600 u, respectively. Find the atomic mass of lithium.
(b) Boron has two stable isotopes, `""_(5)^(10)B` and `""_(5)^(11)B`. Their respective masses are 10.01294 u and 11.00931 u, and the atomic mass of boron is `10.811 u`. Find the abundances of `""_(5)^(10)B and ""_(5)^(11)B`.

Explanation

(a) 6.941 u (b) 19.9%, 80.1%