(55 km/h) * (1 mi)/(1.609344 km) ≈ 34.2 mi/h
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You can do units conversions pretty easily by multiplying by a conversion factor whose value is 1. That is, since
.. 1 mi = 1.609344 km
dividing by the left side we have
.. (1 mi)/(1.609344 km) = 1
This factor has the convenience that it will cancel km and give mi when multiplied by some number off km. That is what we did above. (The "per hour" just went along for the ride. It did not get involved in the units conversion.)
