A double$-$pipe parallel$-$flow heat exchanger is used to heat water $($cp $=4180$ J$/$kg$$K$)$ from $25$C to $60$C at a rate of $0\mathrm{.}2$ kg$/$s$.$ The heating is accomplished by geothermal water $($cp $=4310$ J$/$kg$$K$)$ available at $124$C at a mass flow rate of $0\mathrm{.}3$ kg$/$s$.$ The inner tube is thin$-$walled and has a diameter of $0\mathrm{.}8$ cm$.$ If the overall heat transfer coefficient of the heat exchanger is $550$ W$/$m$2$K$,$ determine the length of the tube required to achieve the desired heating. $($Round the answer to a single decimal place.$)$