Compute: $\dfrac{1}{3} - \dfrac{2}{5}$.

Question

Accepted Answer

B. $-\dfrac{1}{15}$

Find a common denominator. LCM$(3, 5) = 15$.
$\dfrac{1}{3} = \dfrac{5}{15}$, $\dfrac{2}{5} = \dfrac{6}{15}$.
$\dfrac{5}{15} - \dfrac{6}{15} = -\dfrac{1}{15}$.
Key detail: $\dfrac{2}{5}$ is **larger** than $\dfrac{1}{3}$ (since $\dfrac{2}{5} = 0.4$, $\dfrac{1}{3} \approx 0.333$), so the answer is **negative**.
- Trap D drops the negative sign.
- Traps B and C don't compute carefully.

Answer

$\dfrac{1}{15}$

Answer

$-\dfrac{1}{2}$

Answer

$\dfrac{1}{2}$