0.5=2x/0.4-x

$${\mathtt{0.5}} = {\frac{{\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{x}}}{{\mathtt{0.4}}}}{\mathtt{\,-\,}}{\mathtt{x}} \Rightarrow {\mathtt{x}} = {\frac{{\mathtt{1}}}{{\mathtt{8}}}} \Rightarrow {\mathtt{x}} = {\mathtt{0.125}}$$