Each of three blocks \( P \), \( Q \) and \( R \) shown in figure has a mass of \( 3 \, \text{kg} \). Each of the wires \( A \) and \( B \) has cross-sectional area \( 0.005 \, \text{cm}^2 \) and Young's modulus \( 2 \times 10^{11} \, \text{N} \, \text{m}^{-2} \). Neglecting friction, the longitudinal strain on wire \( B \) is _____ \( \times 10^{4} \). (Take \( g = 10 \, \text{m} / \text{s}^{2} \) )
