Explanation:
when comparing fractions (and that includes things like adding our subtracting them) it always helps to bring them on the same level of the denominator (the bottom parts).
in this case we want to bring both fractions to the denominator of 10, because this is the smallest common multiple between 5 and 10.
ah, what do I need to do to convert 5 into 10 ? I need to multiply by 2.
and to keep the overall value of the fraction I need to multiply then the numerator with the same factor.
so,
2/5 = 2/5 × 2/2 = (2×2)/(5×2) = 4/10
now the calculation looks way easier
-9/10 - 4/10 = -13/10 = -1 3/10
so, A is the correct answer.