This is the boss. No calculator, and the numbers are chosen to fit in your head. One motor, four lines, and every answer feeds the next: field speed from the supply, slip from the strobe reading, input power from the efficiency, line current from the input power. Write each line down as you go — one early slip rolls all the way to the feeder.

The last line hides a pleasant accident worth knowing for life. A power factor of 0.866 is $\cos 30^\circ$, which is $\dfrac{\sqrt{3}}{2}$ — so $\sqrt{3}\,V\,\mathrm{PF} = \sqrt{3}\cdot\dfrac{\sqrt{3}}{2}\cdot V = 1.5V$ exactly. At 400 V that is 600 watts per amp, and the whole three-phase calculation collapses into a division you can do standing at a starter panel.

Everything in the chapter rides in this one machine. 5★ under par here is the chapter crown. Clock in.