Average atomic mass of an element is a sum of the product of the isotope mass and its relative abundance.
For example: Chlorine has 2 isotopes with the following abundances
Cl(35): Atomic mass = 34.9688 amu; Abundance = 75.78%
Cl(37): Atomic mass = 36.9659 amu; Abundance = 24.22 %
Average atomic mass of Cl = 34.9688(0.7578) + 36.9659(0.2422) =
= 26.4993 + 8.9531 = 35.4524 amu
Thus, the term “ average atomic mass “ is a <u>weighted</u> average so it is calculated differently from a normal average
Answer:- 
Explanations:- The first reaction is combustion reaction as the hydrocarbon is burned in presence of oxygen to give carbon dioxide and water.
Second reaction is the synthesis reaction as in general a synthesis reaction looks like:

Third reaction is the decomposition reaction as it looks opposite of synthesis reaction.

In general, a single displacement reaction looks like:

Fourth reaction is single replacement reaction as bromine is replaced by chlorine.
So, the correct choice is the last reaction,
.
Answer:
All ions must a overall charge
Explanation: