Any increment of the beat can be notated as either rests or notes.
Each and every complete measure must always add up to the total number of beats defined by the time-signature. Measures are filled with a combination of sounds that you produce and the silences between them.
Rests can indicate silence for portions of Triplets too!
Later on, you will learn partial measures and additional advanced rhythmic devices. For now, the goal is to teach you rhythm basics and prepare to master the Rhythm Exam!
When practicing for the Rhythm Exam, the only competition that a student should experience is with their self. Rhythm skills must be learned by everyone in order to play as a unified ensemble.