Although 'equals' is now accepted the model answer still says 'equal'. In English this would be the case if you were clearly talking about a number of things.
Conventionally if you were doing this as maths (as this appears) in UK English you would always say equals.
I have never heard "equal" being used like this in my life. I thought it must just be a typo! Of course ten minus four equals six.
I have seen use of fanno in this case in some written material. dieci meno sei fanno quattro". which is more common? Is it wrong to use fanno?
I would never say "fanno" in this context. Maybe it's incredibly formal or not related to sums et similars?
It's just because we do seem to treat the numbers as plural (at least to do with hours and so forth) that I can understand the wish to use the 'they make' form rather than the 'it makes' form. But certainly here on its section about math http://italian.about.com/library/fare/blfare135a.htm it's 'fa' all the way.
Perhaps we use "it" instead of "they" because the plus/minus joins the numbers together as a single term, i.e. (ten minus four), it makes six