How many stereo isomers does this molecule have?
${\mathrm{CH}}_3-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_2-\mathrm{CHBr}-{\mathrm{CH}}_3$

The number of stereo isomers is the sum of geometrical isomers (because of C = C) and optical isomers
(because of pressure of Chiral carbon atom)
Number of G.I = 2 ( One C = C is present )
Number of O.I = 2 ( One chiral carbon atom )
$\therefore$ Total number of stereo isomers = 2 + 2 = 4