According to both my dictionary and my experience, the correct classifier for 電腦（电脑）is 台/臺. Of course that hasn't been taught yet, but it should still be accepted, especially to avoid punishing people taking the placement test.
Has now been added as an accepted translation.
Since hen is 'very', why isn't it just gui and not hengui?
Hen is used more often as a way to connect a noun with an adjective, since shi isn't used.
hen is used to connect subject to adjectives.