In the reaction, P+Q ? R+S The time taken for 75 \% reaction of P...

For $\mathrm{P}$, if $\mathrm{t}_{50 \%}=\mathrm{x}$ then $\mathrm{t}_{75 \%}=2 \mathrm{x}$ This is true only for first order reaction. So, order with respect to $\mathrm{P}$ is $1.$ Further the graph shows that concentration of $\mathrm{Q}$ decreases with time. So rate, with respect to $\mathrm{Q},$ remains constant. Hence, it is zero order wrt Q. So, overall order is $1+0=1$