[tex](R_1 +R_2)||R_3\\\\R_{eq} = \left(\dfrac 1{R_1 + R_2} + \dfrac 1{R_3}} \right)^{-1}\\\\\\~~~~~~=\left( \dfrac{1}{100+200} + \dfrac 1{600} \right)^{-1}\\\\\\~~~~~~=\left( \dfrac{1}{300} + \dfrac{1}{600} \right)^{-1}\\\\\\~~~~~~=\left(\dfrac{3}{600} \right)^{-1}\\\\\\~~~~~~=\dfrac{600}3 \\\\\\~~~~~~= 200~ \Omega[/tex]
