Problem #WSP-5328

Problem

Proposed by USA for IMO 1993. For positive real numbers $a, b, c, d$ prove that $\frac{a}{b + 2 c + 3 d} + \frac{b}{c + 2 d + 3 a} + \frac{c}{d + 2 a + 3 b} + \frac{d}{a + 2 b + 3 c} \geq \frac{2}{3} .$

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