Answer: Question 25 has some illustrations of the various figures. The traditional way of indicating chord inversions derives from the baroque practice of "figured bass," a kind of shorthand writing in which a composer would write a bass line and indicate with numbers what harmonies needed to appear above the bass (this is also discussed in more detail in Exploring Theory with Practica Musica).

For example, a "5" and a "3" below a bass note would indicate that this bass should be the root of a triad (which contains the 5th and the 3rd above that bass note). But figured bass was usually further abbreviated; 5/3 being so common it was just assumed if not written. A chord in "first inversion," with its 3rd in the bass position, would have a 6/3: for example, if the bass is C, a 6th above that is A, and a third above is E, producing an A minor chord in first inversion. "6/3" was usually abbreviated to "6," so "6" still refers to a first inversion chord. "6/4" would be a second inversion triad, such as bass C combined with A and F. And there are others that are used for 7th chords, too.

But for something not mentioned in question 25: You don't want to mix up that kind of notation with modern references like "C6," which in pop or jazz would refer to the chord of the added sixth, e.g. C-E-G-A. Inversion figures derived from figured bass are mostly seen in music theory class, often combined with Roman numerals to indicate the chord root: "IV6" would be a first inversion triad whose root is the 4th degree of the scale.