What is the average atomic mass of bromine if it has 49.7% ⁷⁹₃₅Br and 50.3% ⁸¹₃₅Br?

A sample of an element A contains two isotopes ¹⁶₈A and ¹⁸₈A. If the average atomic mass of the element is 16.2 amu, calculate the percentage of the two isotopes in this sample.

Assertion (A): The atomic mass of an element is mainly the mass of its nucleus.

Reason (R): The nucleus of an element contains neutrons and protons and the electrons have a negligible mass.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.