Converting units
Multiply by a fraction that equals 1, arranged so the unit you do not want cancels.
A worked example
Convert 2.5 km to metres. There are 1000 m in 1 km, so 2.5 km x (1000 m / 1 km) = 2500 m. The km cancels top and bottom, leaving metres.
The mistake people actually make
Write the unit you want on top. If you get a number that looks far too big or small, you likely flipped the fraction.