$18\mathrm{.}1$ A household iron has a stainless$-$steel sole plate that weights $3lb$ and has a surface area of $0\mathrm{.}5f{t}^2.$ The iron is rated at $500W.$ If the surroundings are at a temperature of $80F,$ and the convective heat$-$transfer coefficient between the sole plate and surroundings is $3Bt\frac{u}{h}f{t}^{2}F,$ how long will it take for the iron to reach $240F$ after it is plugged in$?$