Let $n$ be the desired the number, so adding the number to the numerator and denominator of $\dfrac{5}{8}$ gives $\dfrac{5+n}{8+n}$, and we must have $$\dfrac{5+n}{8+n} = 0.4.$$  Writing $0.4$ as a fraction, we have $$\dfrac{5+n}{8+n} = \dfrac{2}{5}.$$Multiplying both sides by $8+n$ and by 5 gives $$5(5+n) = 2(8+n).$$Expanding both sides gives$$25+5n = 16 + 2n.$$Subtracting 25 and $2n$ from both sides gives $3n=-9$, so $n=\boxed{-3}.$
What number, when added to the numerator and to the denominator of $\frac{5}{8}$, results in a fraction whose value is 0.4?
{'answer': '-3', 'subject': 'Algebra', 'level': 2, 'unique_id': 'test/algebra/2695.json'}