It is important to realise that in Mathematics it’s not about the answer but about seeing and following the patterns to get to the answer. In other words, method is king.
Unfortunately, in primary school it happens that learners mostly get only 1 marks per question. For example, the question will be about how many months in the year have 31 days. And then the teacher leaves a small line for the answer and type the next question. This give the learner the idea that he must get the answer from his brain somehow and just write down the number.
Therefore, my opinion is that “fill in” question papers are bad – real bad!
Give them a question paper and let them write the test on lined paper. Let them use as many pages as they like. Why not? What have you got against a separate stapled answer sheet? To give the learner the ability to think, to set out the problem and then to get to the correct answer overrides any objections you may have.
How would I answer the test question?