How to enter compound units?

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You can do something like:

sage: a = 3 * units.length.kilometer() / units.time.hour() + 12 * units.length.meter()/units.time.second()
sage: a.convert()
77/6*meter/second

But be careful that the current units system is able to add incompatible units and also forget the unit when some terms cancel, see for example this ask question.