This is a method of converting different units.
The units are first written as a unit and corresponding conversion.
The unit can be written on top, or on bottom
They are then laid out top to bottom, so that they can be cancelled as in multiplying fractions.
Examples
Convert hours to Millennia
This can be extended to anything that has a rate of 1 to something.
To convert from kilometres per hour to metres per second
since there are a thousand metres in a kilometre , sixty seconds in a minute and sixty minutes in an hour.
To convert from metres per second to kilometres per hour
To convert from miles to kilometres
To convert from kilometres to miles
or