Constant v per hz speed: PE Power Rotating Machines practice question

A 10 hp, 460 V, 60 Hz, 4-pole induction motor has a nameplate full-load speed of 1,740 rpm. It drives a constant-torque belt conveyor that loads the motor to its rated torque at every speed. A variable-frequency drive slows the conveyor by running the motor at 30 Hz, holding the volts-per-hertz ratio constant (230 V at 30 Hz).

The motor shaft speed at 30 Hz is most nearly: