Additional Information
Only in the imperial system of measures are mixed units "allowed". That is, measurements like19 feet 4 inches 17 miles 13 yards 2 feet 5 pounds 7 ounces 3 tons 5 hundredweights 2 stones etcIn the SI or metric system mixed measures are "not allowed", and they are certainly not necessary. Something like15 metres 78 centimetres 3 millimetresis expressed as15.783 metresand it is so easy with a system which is based on powers of 10. Just a matter of adjusting the decimal point. It can be done on sight, no arithmetic is necessary. Contrast that with trying to do a similar thing in the imperial system. But do we need to be able to change mixed units into a single unit? Most definitely! Costing depends upon it. So does finding areas and volumes. Using formulas and changing into other systems also require it. So, science, engineering and commerce have to deal with it regularly. It is no wonder that science "went metric" over a 100 year ago.
Perhaps it should be added that, although reference has been made here to the awkward nature of the imperial system, the measuring systems of nearly all other cultures and times have the same underlying fault in their mixed systems 

We should be clear where the "fault" lies. It is not in the fact that the imperial system uses measures called pounds and ounces etc., but in the relationships between those measures.16 ounces = 1 pound 14 pounds = 1 stone 8 stones = 1 hundredweight etc.Now consider the measurement of5 stones 2 pounds 7 ouncesTo change that into a single measure (for whatever purpose) some arithmetic is needed to get any one of these5.174107 stones OR 72.4375 pounds OR 1159 ouncesIF instead we had10 ounces = 1 pound 10 pounds = 1 stonethen the above mixed measurement could easily be read off as any of5.27 stones 52.7 pounds 527 ouncesNo contest! 