a, y + 30%y = -1,3
1.y + 30/100y = -13/10
1.y + 3/10y = -13/10
(1 + 3/10). y = -13/10
13/10 .y = -13/10
y = -13/10 : 13/10
y = -13/10 . 10/13
y = -1
b, y - 25%y = 1/2
1.y - 25/100.y = 1/2
1.y - 1/4.y = 1/2
(1 - 1/4) y = 1/2
3/4 .y = 1/2
y = 1/2 : 3/4
y = 1/2 . 4/3
y = 2/3
câu c mik chưa nghĩ ra