You're working as a Mechanical Engineer tasked with modelling the efficiency, $,$

of a heat engine. This efficiency depends on variables such as the engine's

temperature $(T),$ pressure $(P),$ and the fuel's caloric value $(C).$ The existing model

provides you with the function:

$(T,P,C)=0\mathrm{.}5T^{2}P+0\mathrm{.}3TPC+0\mathrm{.}2P{C}^2$

To find the rate of change of efficiency with respect to pressure when the

temperature is increased by $1$ degree and fuel's calorie value decreases by $1$ unit.