Подставим вместо неизвестного числа х и решим уравнение.
8 + x = 35 - 20
8 + x = 15
x = 15 - 8
x = 7
8 + 7 = 35 - 20
9 + x = 57 - 40
9 + x = 17
x = 17 - 9
x = 8
9 + 8 = 57 - 40
6 + x = 24 - 10
6 + x = 14
x = 14 - 6
x = 8
6 + 8 = 24 - 10
x - 8 = 36 - 30
x - 8 = 6
x = 6 + 8
x = 14
14 - 8 = 36 - 30
x - 7 = 49 - 43
x - 7 = 6
x = 6 + 7
x = 13
13 - 7 = 49 - 43
x - 5 = 18 - 9
x - 5 = 9
x = 9 + 5
x = 14
14 - 5 = 18 - 9
18 - x = 29 - 20
18 - x = 9
x = 18 - 9
x = 9
18 - 9 = 29 - 20
12 - x = 60 - 56
12 - x = 4
x = 12 - 4
x = 8
12 - 8 = 60 - 56
11 - x = 95 - 90
11 - x = 5
x = 11 - 5
x = 6
11 - 6 = 95 - 90
3 1/6 - (× + 11/12) = 1/4
х + 11/12 = 3 1/6 - 1/4
× + 11/12 = 3 2/12 - 3/12
× + 11/12 = 2 14/12 - 3/12
× + 11/12 = 2 11/12
× = 2 11/12 - 11/12
× = 2
6 5/27 - (х - 1 2/9) = 3 20/81
× - 1 2/9 = 6 5/27 - 3 20/81
× - 1 2/9 = 6 10/81 - 3 20/81
× - 1 2/9 = 5 91/81 - 3 20/81
× - 1 2/9 = 2 71/81
× = 1 2/9 + 2 71/81
× = 1 18/81 + 2 71/81
х = 3 89/81 = 4 8/81
3 5/36 -( 1 4/9 - х) = 1 17/18
1 14/9 - х = 3 5/36 - 1 17/18
1 14/9 - х = 3 5/36 - 1 34/36
1 14/9 - х = 2 41/36 - 1 34/36
1 14/9 - х = 1 7/36
х = 1 14/9 - 1 7/36
х = 1 56/36 - 1 7/36
× = 49/36 = 1 13/36
P.S. я не уверена на счет 3 примера)
90=60+0,1*Х
0,1*Х=30
Х=300грамм
ответ добавили 300 граммов воды