$5\mathrm{.}44.$ A nuclear power plant generates $750MW$; the reactor temperature is $315C$ and a river with water temperature of $20C$ is available.

$($a$)$ What is the maximum possible thermal efficiency of the plant, and what is the minimum rate at which heat must be discarded to the river?

$($b$)$ If the actual thermal efficiency of the plant is $60\%$ of the maximum, at what rate must heat be discarded to the river, and what is the temperature rise of the river if it has a flow rate of $165m^3*s^{-1}?$