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