\[\boxed{\text{51\ (51).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\frac{4a^{2} - ab}{a + 14b^{2}} = \frac{a(4a - b)}{b(a + 14b)}\]
\[если\frac{a}{b} = 5;\ \ то\ a = 5b:\]
\[\frac{5b(4 \cdot 5b - b)}{b(5b + 14b)} = \frac{95b^{2}}{19b^{2}} = \frac{95}{19} = 5.\]