Помогите решить 4) и 5) срочно отношение дробей

Решите задачу:

$\displaystyle \tt 4). \ \ b:\frac{25}{6}=\frac{4}{7}:\frac{20}{21}\\\\{} \ \ \ \ \ \ b:\frac{25}{6}=\frac{4}{7}\cdot\frac{21}{20} \\\\{} \ \ \ \ \ \ b=\frac{3}{5}\cdot\frac{25}{6}\\\\{} \ \ \ \ \ \ b=\frac{5}{2}\\\\{} \ \ \ \ \ \ b=2,5\\\\\\5). \ \ \frac{x+3}{7}=\frac{2}{5}\\\\{} \ \ \ \ \ \ x+3=\frac{2}{5}\cdot7\\\\{} \ \ \ \ \ \ x=2,8-3\\\\{} \ \ \ \ \ \ x=-0,2$