Two separate soap bubbles of radii $3\times {10}^{-3} \mathrm{m}$ and $2\times {10}^{-3} \mathrm{m}$ respectively, formed of same liquid (surface tension $65\times {10}^{-2} \mathrm{N} {\mathrm{m}}^{-1}$) come together to form a double bubble. The radius of interface of double bubble is

A mild steel wire of length $2 L$ and cross-sectional area $A$ is stretched, well with in the elastic limit, horizontally between two pillars as shown in figure. A mass $m$ is suspended from the mid-point of the wire. Strain in the wire is