\(A=\frac{\frac{1}{6}-\frac{1}{39}+\frac{1}{51}}{\frac{1}{8}-\frac{1}{52}+\frac{1}{68}}=\frac{\frac{1}{3}\cdot\left(\frac{1}{2}-\frac{1}{13}+\frac{1}{17}\right)}{\frac{1}{4}\cdot\left(\frac{1}{2}-\frac{1}{13}+\frac{1}{17}\right)}=\frac{1}{3}:\frac{1}{4}=\frac{4}{3}\)