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