The resistances of iron and copper wires at $20° \mathrm{C}$ are $3.9 \mathrm{\Omega }$ and $4.1 \mathrm{\Omega }$ respectively. At what temperature will the resistances be equal? Temperature coefficient of resistivity for iron is $5.0\times {10}^{-3}{\mathrm{K}}^{-1}$ and for copper it is $4.0\times {10}^{-3}{\mathrm{K}}^{-1}$. Neglect any thermal expansion.