Question

how combination reaction related to decomposition reaction

Answer 1

Decomposition reactions are just opposite of combination reactions.

Example- 2Hg(Mercury) + O2 ------› HgO
2HgO ------› 2Hg +O2

Answer 2

Combination reaction is just related to Decomposition reaction because in combination reaction substances combined to form single substances.
E. G. : 2Mg+O2 ----->2Mgo

But In case of Decomposition reaction substances were spilt up in one or more simpler form of substance
E. G. : 2H2o----->Co2 +2 H2

I hope it will help you..!!