Double bond equivalent is used to find the level of unsaturations present in an organic molecule.

DBE =

Where C= number of carbon atoms present

H=number of hydrogen atoms present

N=number of nitrogen atoms present

X=number of halogen atoms present

Double bond equivalent (DBE) for C₄H₉Br

=

=0

So none of the isomers has a ring or unsaturation, so the isomers are positions or chain isomers as shown below in the table: