Solve the telescope triangle: focal ratio = focal length ÷ aperture. Enter any two and get the third.
Focal ratio = focal length ÷ aperture. A 1200 mm scope with 150 mm aperture is f/8. Fast scopes (f/4-f/5) give wide bright fields for deep sky and short exposures; slow scopes (f/10+) give comfortable high magnification for planets and the Moon.
The focal length divided by the aperture: 1200 mm of focal length through a 150 mm aperture is f/8. It describes the light cone, exactly like the f-number of a camera lens.
Neither, they suit different jobs. Fast (f/4-f/5) means a wider, brighter field and shorter astrophoto exposures but harder-to-make optics and more coma. Slow (f/10+) means narrow bright views that excel on planets, and it is gentler on cheap eyepieces.
For extended objects the exit pupil (aperture ÷ magnification) sets surface brightness at the eye, not the f-ratio itself. Where f-ratio directly matters is imaging exposure time, which scales with the square of the ratio.