Although the student in the example used in this question did not make
too many errors, there is a better way of working out this equation so
that you will find the correct answer. In an equation like this where
you need to get I on it's own, you need to divide everything by two to
get -w+P over 2 = l. To get to this point, to start with you would need
to do -2w+P=2l instead of P-2w=2l.
