How does the computer know which parts are correct

when i check my work, how does the computer know wich parts are corect and which are missing, and why cant he do all de work by his self if he knows the missing parts?

 

For the tutorial, we have the correct answer stored so that we can check your work.  For the real game you are working without a net since we have no way of knowing what the right answer is.  For that we compare what you do with what everyone else does.