COST \[C=av+\frac{b}{v}\] ACCORDING to given question, \[30a+\frac{b}{30}=75\] ?(i) \[40a+\frac{b}{40}=65\] ?(ii) On solving (i) and (ii), we get \[a=\frac{1}{2}\] and \[b=1800\] Now, \[C=av+\frac{b}{v}\Rightarrow \frac{DC}{dv}=a-\frac{b}{{{v}^{2}}}\] \[\frac{dC}{dv}=0\Rightarrow a-\frac{b}{{{v}^{2}}}=0\Rightarrow v=\sqrt{\frac{b}{a}}=\sqrt{3600}\] \[\Rightarrow v=60\,\,kmph\]