IT = (vT – v1)/20 + (vT – 40i∆)/60
(v1 – 40v∆)/16 + v1/80 + (v1 – vT)/20 = 0
10v1 - 200i∆ = 4vT i∆ = -v1/80, 200 i∆ = -2.5v1
12.5v1 = 4vT; v1 = 0.32vT
i∆ = - v1/80 = -0.32vT /80
40i∆ = - 0.16vT
iT = (vT – 0.32vT)/20 + (vT + 0.16vT)/60
= ( (0.68 vT)(3) + 1.16vT ) /60
= ( (2.04 + 1.16)vT )/60 = 3.20vT/60
VT /iT = 60/3.2 = 18.75 Ω
è RTh = 18.75 Ω