Start with the identity property of multiplication.
anything multiplied by 1 is equal to itself.
The rule of divisibilty
any number divided by itself is equal to 1
Let’s convert 20 meters to feet. We need a conversion factor:
by the rule of divisiblity:
note that I put the units I want to end with (feet) on top.
we know that
and since we can do this:
the meters cancel out:
and all we are left with is the multiplication:
As long as you have a conversion factor between the two things you are converting you can do this. You are always multiplying by 1 and canceling out the units of measure. It gets kind of mechanical.