Decompostion reaction have the general chemical reaction of: AB = A + B while synthesis reactions are of the general equation A + B = AB. These reaction can be the reverse of each other because ones building a compound (synthesis) and ones breaking down a compound (decomposition). For example, when reacting carbon and oxygen for a synthesis reaction we have: C + O2 = CO2 and for a decomposition reaction we have: CO2 = C + O2. Thus, these two reactions are the reverse of each other.
