Calculate data used streaming video and music.
Combining video and audio in one calculation makes the scale difference impossible to miss. Video at 5 Mbps costs about 2.2 GB an hour; music at 256 kbps costs about 0.11 GB. Sixty hours of video therefore outweighs forty hours of music by a factor of thirty, which is why any serious attempt to reduce streaming data has to start with video quality and never with audio.
Combined streaming volume
Video GB = Mbps x 3600 / (8 x 1024) x video hours; music GB = kbps x 3600 / (8 x 1024 x 1024) x music hours; total = video + music
Quality, usually. Dropping from 4K to 1080p cuts that traffic by about two thirds without watching any less, whereas cutting hours means giving something up. Both scale linearly, so halving either halves the video figure.
Only if you stream them. A downloaded title is transferred once regardless of how long it plays, so downloads are better estimated from file size than from viewing hours.
Purely convention reflecting scale. Audio bitrates are three orders of magnitude smaller, so kilobits keeps the numbers readable — 256 kbps rather than 0.256 Mbps.