Let's solve each equation one by one:

A: 45 - 9X = 42
First, subtract 45 from both sides:
-9X = 42 - 45
-9X = -3
Next, divide both sides by -9 to solve for X:
X = -3/-9
X = 1/3

B: (5X - 9)/3 = 12
First, multiply both sides by 3 to get rid of the fraction:
5X - 9 = 36
Next, add 9 to both sides:
5X = 36 + 9
5X = 45
Finally, divide both sides by 5 to solve for X:
X = 45/5
X = 9

C: X - 87/29 = 3
First, multiply both sides by 29 to get rid of the fraction:
29(X) - 87 = 3 * 29
29X - 87 = 87
Next, add 87 to both sides:
29X = 87 + 87
29X = 174
Finally, divide both sides by 29 to solve for X:
X = 174/29
X = 6

D: 70 - (X - 3) = 45
First, distribute the negative sign to the terms inside the parentheses:
70 - X + 3 = 45
Next, combine like terms:
73 - X = 45
Next, subtract 73 from both sides:
-X = 45 - 73
-X = -28
Finally, multiply both sides by -1 to solve for X:
X = -28/-1
X = 28

E: X - 93 = 175 : 25
First, divide 175 by 25:
X - 93 = 7
Next, add 93 to both sides:
X = 7 + 93
X = 100

F: 21 / (X - 1) = 3
First, multiply both sides by (X - 1) to get rid of the fraction:
21 = 3(X - 1)
Next, distribute the 3 to the terms inside the parentheses:
21 = 3X - 3
Next, add 3 to both sides:
24 = 3X
Finally, divide both sides by 3 to solve for X:
X = 24/3
X = 8

G: 30 - 3(X - 2) = 12
First, distribute the 3 to the terms inside the parentheses:
30 - 3X + 6 = 12
Next, combine like terms:
36 - 3X = 12
Next, subtract 36 from both sides:
-3X = 12 - 36
-3X = -24
Finally, divide both sides by -3 to solve for X:
X = -24/-3
X = 8

So the solutions for each equation are:
A: X = 1/3
B: X = 9
C: X = 6
D: X = 28
E: X = 100
F: X = 8
G: X = 8