Решим уравнения по порядку:
1) $$x - 890 \cdot 20 = 1050$$
$$x - 17800 = 1050$$
$$x = 1050 + 17800$$
$$x = 18850$$
2) $$300 + c = 3000 : 2$$
$$300 + c = 1500$$
$$c = 1500 - 300$$
$$c = 1200$$
3) $$y : (1098 - 998) = 4$$
$$y : 100 = 4$$
$$y = 4 \cdot 100$$
$$y = 400$$
4) $$x + 880 : 20 = 1050$$
$$x + 44 = 1050$$
$$x = 1050 - 44$$
$$x = 1006$$
5) $$3000 + c = 6000 : 2$$
$$3000 + c = 3000$$
$$c = 3000 - 3000$$
$$c = 0$$
6) $$(1098 - 998) : y = 4$$
$$100 : y = 4$$
$$y = 100 : 4$$
$$y = 25$$
Ответы:
$$x = 18850$$
$$c = 1200$$
$$y = 400$$
$$x = 1006$$
$$c = 0$$
$$y = 25$$