In America, we do that with unit multipliers.

We write it over 1, so it will have the form of a fraction:



We make a unit multiplier, which is a fraction with 'dollars' 
on the bottom and 'pounds' on the top.  We put 'dollars' on the
bottom of the unit multiplier so that it will cancel with the
'dollars' on the top of the beginning fraction.

We use the fact that she can buy 1.50 dollars for 1 pound. So
our unit multiplier is the fraction .  So multiplying by that:

 '
Now you see that the words (units) 'dollars' cancel out like this:



and the only leave pounds in the numerator. And we have:



and then



and then



and then



and we can put an "s" on "pound"



and write with a pound symbol, just like they
do over across the pond.  :)

£200

So $300 costs £200. :)

Edwin