If e.g. c
is the speed of light:
c = UnitConvert[Quantity["SpeedOfLight"]]
Then the obvious way would be to write:
c/Quantity["Meters"/"Seconds"]
Because that way, you're certain the unitless quantity you work with actually means something in m/s, not e.g. ft/hour. In other words: If you wrote c/Quantity["Feet"/"Hours"]
, the result would be the speed of light, in ft/hour, without a unit attached.
Alternatively, you could always write:
QuantityMagnitude[c]
Which just returns the magnitude, without the unit