How many geometrical isomers does hexa-2,4-diene, $\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}{}_3$, have?

Each $\mathrm{C}=\mathrm{C}$ can be $E$ or $Z$, giving $2^2=4$ combinations $(E,E)$, $(Z,Z)$, $(E,Z)$, $(Z,E)$. The molecule is symmetric, so $(E,Z)$ and $(Z,E)$ are the same compound. The count is therefore $3$.