Thevenin Voltage and Resistance Calculations
For \$\small V_{TH}\$, let \$\small V\$ be the 25/55/25v node, then: \$\frac{V-4I_x}{29}+\frac{V}{55}+\frac{V-25}{20}=0\$, and \$I_x=\frac{V}{55}\$. Hence \$\small V_{TH}=V-25\$.