$14\mathrm{.}16$ A $240-V$ series motor has an armature resistance of $0\mathrm{.}42$ and a series$-$field resistance of $0\mathrm{.}18.$ If the speed is $500\frac{r}{m}in$ when the current is $36$ A $,$ what will be the motor speed when the load reduces the line current to $21$ A $?($Assume a $3-V$ brush drop and that the flux is proportional to the current.$)$