Which should be subtracted from $2\dfrac{1}{2}$ to get $-3\dfrac{1}{3}$ ?

Let $x$ be subtracted from $2\dfrac{1}{2}$.

$2\dfrac{1}{2} - x = -3\dfrac{1}{3}\\[1em] \Rightarrow \dfrac{5}{2} - x = -\dfrac{10}{3}\\[1em] \Rightarrow x = \dfrac{5}{2} - \dfrac{-10}{3}$

LCM of 2 and 3 is 2 x 3 = 6

$\Rightarrow x = \dfrac{5 \times 3}{2 \times 3} - \dfrac{-10 \times 2}{3 \times 2}\\[1em] \Rightarrow x = \dfrac{15}{6} - \dfrac{-20}{6}\\[1em] \Rightarrow x = \dfrac{15 - (-20)}{6}\\[1em] \Rightarrow x = \dfrac{15 + 20}{6}\\[1em] \Rightarrow x = \dfrac{35}{6}\\[1em] \Rightarrow x = 5\dfrac{5}{6}$

The number subtracted from $2\dfrac{1}{2}$ to get $-3\dfrac{1}{3}$ is $5\dfrac{5}{6}$.