R = r + r + r + \frac{(r / 2) \cdot 2 r}{r / 2 + 2 r} = 30 + \frac{100}{25} = 34 Ω

${R = r + r + r + \frac{(r/2) \cdot 2r}{r/2+2r} = 30 + \frac{100}{25} = 34 \Omega}$

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