Which of the following is true about sodium chloride?

An element has three isotopes and their isotopic weights are $11, 12, 13$ units,and their percentage of occurrence in nature is $80 \%, 15 \%, 5 \%$ respectively. Then the average atomic weight of the element will be $:-$

Naturally occurring boron has $20\%$ $_5B^{10}$ and $80\%$ $_5B^{11}$. The average atomic weight of boron is

$1 \ g$ of hydrogen combines with $80 \ g$ of bromine. $1 \ g$ of calcium (valency $= 2$) combines with $4 \ g$ of bromine. The equivalent weight of calcium is ...

Which of the following statements is not true about atomic mass?

Assuming chlorine element has two isotopes as $Cl^{35}$ and $Cl^{37}$ with their percentage abundance as $25\%$ and $75\%$ respectively. Find the average atomic weight.