How on earth can you do math with that sort of stuff? You need to write it out as fractions.
A big clue is 'per' or '/' or 'every' or 'a'. These mean the same thing. They mean that the units before that word go on top of the equation and the units after it go on the bottom.
It doesn't matter how many 'pers' or slashes there are in the statement. The units after them go on the bottom of the fraction.
Sometimes a list will give you the units first and then the number. If it does this, the number goes on top in your fraction.
One of the most confusing things is when the information isn't a fraction! What do you do with 'the client's weight = 72 kg'?
Think of it as 
,
if that makes it easier. But you could also treat it as the first thing you
know about the case, the way it is done in the examples on the conversion
factors page. You can think of it as the initial information you need to
convert into the answer. For instance, you might know the client's weight
but need to convert that into mg of medication.