A student accidentally added five to both the numerator and denominator of a fraction, changing the fraction's value to $\frac12$. If the original numerator was a 2, what was the original denominator?
I know that the original fraction had a 2 in the numerator, so let me write it as $\frac{2}{d}$, where $d$ is the unknown denominator. Then the student added five to both the numerator and denominator, getting $\frac{2+5}{d+5} = \frac{7}{d+5}$. This new fraction is equal to $\frac12$, so I can write an equation: $\frac{7}{d+5} = \frac12$. To solve for $d$, I can cross-multiply and get $7 \cdot 2 = (d+5) \cdot 1$, which simplifies to $14 = d+5$. Subtracting 5 from both sides, I get $d = 9$. So the original denominator was 9.

