12 Tone Equal Temperament (12TET)

Interval: The relationship between one note and another expressed in distance, or frequency.
Octave: An interval of a lower and higher version of the same note. Example: A4(440hz) and A5(880hz).

Multiply A4(440hz) twelve times by what number to arrive at its Octave A5(880hz)?

The 12th Root of 2 = 12√2 = 21/12 = 1.0594630943592953...

Interesting Links:
Equal Temperament Formula Michigan Tech University - Physics
Equal Temperament Explained Georgia State
Acoustic Ecology Comparison of 12TET to Pythagorean to Just Intonation