55 Реши уравнение: 1) \frac{450}{x}=9; 2) \frac{y}{7}-9; 3) \frac{9+a}{9}=23; 4) \frac{504}{b}-18=72.

1. $$\frac{450}{x}=9$$
   $$x = \frac{450}{9}$$
   $$x = 50$$
2. $$\frac{y}{7}=9$$
   $$y = 9 \cdot 7$$
   $$y = 63$$
3. $$\frac{9+a}{9}=23$$
   $$9+a = 23 \cdot 9$$
   $$9+a = 207$$
   $$a = 207 - 9$$
   $$a = 198$$
4. $$\frac{504}{b}-18=72$$
   $$\frac{504}{b}=72+18$$
   $$\frac{504}{b}=90$$
   $$b = \frac{504}{90}$$
   $$b = \frac{252}{45}$$
   $$b = \frac{84}{15}$$
   $$b = 5\frac{9}{15}$$
   $$b = 5\frac{3}{5}$$

Ответ: 1) 50; 2) 63; 3) 198; 4) $$5\frac{3}{5}$$.