Hmm . . . let's see. It may well be that これ is a typo or an imperfect recollection, but if I had to mount a defence of it, my argument would go like this: in circumstances where it is essential that a text such as a law, for example, should be absolutely unambiguous, it is common to recapitulate subject or object with これ so as to be quite clear, even though the same passage without これ would be regarded as acceptable in ordinary expository prose. There are numerous examples, for instance, in the Japanese Constitution. Here are a couple of reasonably straightforward ones:
In order to achieve the object of the preceding section, an army, navy and air force and other war potential - we will not maintain these.
The right of the state to engage in warfare - we will not recognise this
(Official,translation: In order to accomplish the aim of the preceding paragraph, land, sea and air forces, as well as other war potential, will never be maintained. The right of belligerency of the state will not be recognised.)
The same convention is used in language textbooks to make clear the structures of sentences in other languages.
If it is used in maths problems as well, which would seem likely, I would interpret 179からある数これを引いたら、残りは12になりました。ある数はいくつですか。 as:
A certain number, if you deduct this from 179, gives a remainder of 12. How much is this certain number?