в) 86/113 : 43/51;

$$\frac{86}{113} : \frac{43}{51} = \frac{86}{113} \cdot \frac{51}{43} = \frac{2 \cdot 43}{113} \cdot \frac{51}{43} = \frac{2 \cdot 51}{113} = \frac{102}{113}$$