I'm going to guess that just adding a few intermediate statements
like

20 - 12 = 8 * 3
8 =/= 24

was probably what the teacher was looking for and that nothing was needed for the comparisons like 497 < 500.

I'm mostly with you but on the other hand, being a bit formal and writing intermediate calculations down is a reasonable habit to develop and not terribly cumbersome.

The following says a variant on the same idea:
http://www.artofproblemsolving.com/Resources/articles.php?page=mistakes