Using a Stroboscope to Make a Propeller Appear Stationary

[rcmango]

Homework Statement

A stroboscope is a light that flashes on and off at a constant rate. It can be used to illuminate a rotating object, and if the flashing rate is adjusted properly, the object can be made to appear stationary.
(a) What is the shortest time between flashes of light that will make a three-bladed propeller appear stationary when it is rotating with an angular speed of 14.4 rev/s?
s

(b) What is the next shortest time?

Homework Equations

The Attempt at a Solution

not sure where to start.

 

[Shooting Star]

When the propellor rotates by 120 deg, it looks the same. The light should flash again by the time the propellor has done this.