[tex]\sf \displaystyle 80=\frac{800\cdot 2\cdot \:x}{100}\\\\\\\sf \frac{800\cdot \:2x}{100}=80\\\\\\\sf \frac{100\cdot \:800\cdot \:2x}{100}=80\cdot \:100\\\\\\\sf 1600x=8000\\\\\\\sf \frac{1600x}{1600}=\frac{8000}{1600}\\\\\\\sf \to \boxed{\sf x=5 \ Meses}[/tex]
OK?
Kevin Davi.