Monstrous Mathematical Move!

$\begin{aligned} A &= \dfrac {5}{14} + \dfrac {10}{21} - \dfrac 25 \\ &= \dfrac 57 \times \dfrac 12 + \dfrac 57 \times \dfrac 23 - \dfrac 25 \\ &= \dfrac 57 \times (\dfrac 12 + \dfrac 23) - \dfrac 25 \\ &= \dfrac 57 \times \dfrac 76 - \dfrac 25 \\ &= \dfrac 56 - \dfrac 25 \\ &= \dfrac {13}{30} \\ B &= \dfrac {5 + 10 - 2}{14 + 21 - 5} \\ &= \dfrac {13}{30} = A \end{aligned}$

$\displaystyle{\frac5{14} + \frac{10}{21} - \frac25 <\stackrel{\mathclap{\mbox{?}}}{=}> \frac{5 + 10 - 2}{14 + 21 - 5}}$ $\displaystyle{\frac5{14} + \frac{10}{21} - \frac25 <\stackrel{\mathclap{\mbox{?}}}{=}> \frac{13}{30}}$ $\displaystyle{\frac{75}7 + \frac{100}7 - 12 <\stackrel{\mathclap{\mbox{?}}}{=}> 13}$ $75 + 100 - 84 <\stackrel{\mathclap{\mbox{?}}}{=}> 91$ $91 = 91$ $\boxed{A = B}$

$\displaystyle A = \frac { 5 } { 14 } + \frac { 10 } { 21 } - \frac { 2 } { 5 }, B = \frac { 5 + 10 - 2 } { 14 + 21 - 5 } \rightarrow A = \frac { 13 } { 30 }, B = \frac { 13 } { 30 }$ .

So, the answer is $\boxed { A = B }$ .