fix solution for the equivalent term selection

Unfortunately, I hat a typo in the fraction term making it
not equivalent to a - ab^2.

This leads to only two correct choices worth 3BE.
Therefore give 0.5 BE for each correctly not chosen term.
This has the weird effect that not doing the exercise at all
results in getting 2BE.
And students with wrong auxiliary calculation getting less points
than those doing nothing.

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