Problem $5$:

A large block of steel, which is initially at a uniform temperature of $30C,$ is suddenly changed to and held at $200C.$ Calculate the temperature at a depth of $20mm$ after $60s.$ For steel, $k=45\frac{W}{mK},$ and $=1\mathrm{.}2{10}^{-5}\frac{m^2}{s}.$