Titan/Rhea = 15.94/4.518 = 3.528/1
Rhea makes 3.528 resolutions for each revolution of Titan. Resonance is 3.528:1 Multiplying by 2 yields:
(3.528 x 2)/(1 x 2) = 7.056/2
(2 x 15.94 = 7.056 x 4.518
31.88 = 31.87
When Rhea and Titan are in conjunction as seen from Saturn and travel in their orbits for 31.88 days, they return into conjunction.