What does Marx mean when he says the workers are exploited in capitalism? What is his formula of justice in fully developed communist society.whats...

According to Karl Marx, workers are exploited in
capitalism because they create value but they are not the ones who get the money that is
paid when that value is bought.

Let's say I own a factory. 
I give you wood and you make it into furniture in my factory.  I get paid more than you
do even though you are the one who actually did the work.  Thus, I am getting rich off
of your work.

Marx does not actually say that capitalism is
unjust.  He actually says it is not unjust.  As a website at Stanford University
says

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Capitalism's dirty secret is that it is not a
realm of harmony and mutual benefit but a system in which one class systematically
extracts profit from another. How could this fail to be unjust? Yet it is notable that
Marx never concludes this, and in Capital he goes as far as to say
that such exchange is ‘by no means an
injustice’.

So I am not sure
what you are asking about in terms of a formula of justice.  My best guess is that it is
"from each according to his abilities, to each according to his
needs."

In this formula, everyone works as hard as they can
and gets all that they need (and nothing more).