Dimensional analysis
Looking at the unit labels can help in choosing what to do with the numbers. For example, to convert 2.3 miles to some number of meters, use the equation "1609.34 meters equals one mile" or 1609.34 meters per mile. Using division, the unit labels will be miles/meters/mile, which ends up as miles2/meter, which is not just the meters. With multiplication, the unit labels will be miles × meters/mile. The miles will cancel, leaving just meters.