As you can see below by taking the C at the bottom of the interval and moving it above the G, the initial interval of a 5th turns into a 4th when turned upside down.
Thus a fifth (number 5) and its inverse or complement, a fourth (number is 4) add up to 9.
Yes we can go through the entire process again once have flipped the interval, but there is an easier way.
Reference : inversion of intervals To invert an interval, place the lower note one octave higher or the highest note an octave lower: In the following tables you can see how an …
Inverted intervals are simply intervals which have been turned upside down. The number of any interval and the number of its inversion always add up to nine. In music, the verb invert means to move the lowest note in a group an octave higher. Where intervals are identified by ratio, (for example, a fifth may be expressed as 3/2), the inversion is determined by reversing the ratio and multiplying by 2, in this case 4/3, which is the ratio of a fourth.
To invert an interval just take the bottom note, and put it on the top!
Inverting perfect intervals. Labeling Interval Inversions Once we know the quality and distance of the original interval, it is very easy to determine the quality and distance of it's inversion.