The bond angle between two hybrid orbitals is 180°. The percentage s-character of hybrid orbital is around

The bond angle 180° means sp hybridisation which has 50% s-character.

$\begin{array}{l}\cos \theta =\left(\frac{s\mathrm{\%}}{s\mathrm{\%}-100}\right)\Rightarrow \cos {180}^{\circ }=\left(\frac{s\mathrm{\%}}{s\mathrm{\%}-100}\right)\\ \Rightarrow -1=\frac{s\mathrm{\%}}{s\mathrm{\%}-100}\Rightarrow \therefore s\mathrm{\%}=50\end{array}$