Example $4\mathrm{.}27$: The milk is heated in cross$-$flow heat exchanger $($ters ak$$ml$$ is$$ degistirici$)$ at a mass flow rate of $1000\frac{kg}{h}$ from $42C$ to $70C.$ The water used for heating the milk enters the heat exchanger at $95C$ and leaves the system at $80C.$ If the heat is emitted from the system to its surroundings is $1$ kW $,$ then find out the mass flow rate of water used to heat the milk. The specific heat of milk is $3\mathrm{.}9\frac{kJ}{C.kg}.$