Resposta :
[tex]\Delta{V}=V_o\cdot{\lambda}\cdot\Delta{\theta}[/tex]
[tex]0,2=100\cdot{\lambda}\cdot(80-0)[/tex]
[tex]0,2=8000{\lambda}[/tex]
[tex]\lambda=2,5\cdot{10}^{-5}^oC^{-1}[/tex]
Então:
[tex]\boxed{\beta=1,25\cdot{10}^{-5}^oC^{-1}}[/tex]
Abraços !