Resposta :
50 reais ----- 150 kw/h
x reais ----- 300 kw/h
150 . x = 50 . 300
150x = 15.000
x = 15.000
----------
150
x = R$ 100
[tex]\large\boxed{\begin{array}{l} \rm{R\$ = kw/h} \\ \\ \rm{ \dfrac{50}{x} = \dfrac{150}{300}} \\ \\ \rm{150x =300 \times 50 } \\ \\ \rm{150x = 15000} \\ \\ \rm{x = \dfrac{1500\diagup\!\!\!\!0}{15\diagup\!\!\!\!0}} \\ \\ \boxed{ \rm{x = R \$\,100}} \end{array}}[/tex]