A compound pipe of diameter d1, d2 and d3 having lengths l1, l2 and l3 is to be replaced by an equivalent pipe of uniform diameter d and of the same length (l) as that of the compound pipe. The size of the equivalent pipe is given by

• A. l/d³ = (l₁/d₁³) + (l₂/d₂³) + (l₃/d₃³)
• B. l/d⁴ = (l₁/d₁⁴) + (l₂/d₂⁴) + (l₃/d₃⁴)
• C. l/d² = (l₁/d₁²) + (l₂/d₂²) + (l₃/d₃²)
• D. l/d⁵ = (l₁/d₁⁵) + (l₂/d₂⁵) + (l₃/d₃⁵)

Answer: (D)