(a) Two stable isotope of `._3Li^6` and `._3Li^7` have respective abundances of `7.5%` and `92.5%`. These isotopes have masses 6.01512 and 7.01600 u respectively. Find the atomic weight of lithium.
(b) Boron has two stable isotopes `._5B^(10)` and `._5B^(11)`. Their respective masses are 10.01294 u and 11.00931 u, and the atomic weight of boron is 10.81 u. Find the abundances of `._5B^(10)` and `._5B^(11)`.

Explanation

atomic weight =weighted average of the isotopes
`=(6.01512xx7.5+7.01600xx92.5)/((7.5+92.5))`
`=(45.1134+648.98)/(100)=6.941 u`