To solve these equations, we'll follow the order of operations (PEMDAS/BODMAS) and isolate the variable X.

g) 280 : (7 + 3 x X) = 4

First, we'll simplify the expression inside the parentheses:
7 + 3 x X = 7 + 3X

Now, we'll substitute this simplified expression back into the equation:
280 : (7 + 3X) = 4

To isolate the variable X, we'll multiply both sides of the equation by (7 + 3X):
280 = 4(7 + 3X)

Distribute the 4:
280 = 28 + 12X

Subtract 28 from both sides:
280 - 28 = 28 + 12X - 28
252 = 12X

Divide both sides by 12:
252/12 = 12X/12
21 = X

Therefore, the solution to equation g) is X = 21.

h) 6 x (28 – 8 x X) = 72

First, we'll simplify the expression inside the parentheses:
28 – 8 x X = 28 - 8X

Now, we'll substitute this simplified expression back into the equation:
6 x (28 - 8X) = 72

To isolate the variable X, we'll divide both sides of the equation by 6:
(28 - 8X) = 72/6
28 - 8X = 12

Subtract 28 from both sides:
28 - 28 - 8X = 12 - 28
-8X = -16

Divide both sides by -8:
-8X/-8 = -16/-8
X = 2

Therefore, the solution to equation h) is X = 2.

i) (X – 15 ) x 3 : 12 = 6

First, we'll simplify the expression inside the parentheses:
(X - 15) x 3 : 12 = 6

Now, we'll multiply both sides of the equation by 12 to get rid of the division:
3(X - 15) = 6 x 12
3X - 45 = 72

Add 45 to both sides:
3X - 45 + 45 = 72 + 45
3X = 117

Divide both sides by 3:
3X/3 = 117/3
X = 39

Therefore, the solution to equation i) is X = 39.

k) (X : 4 + 6) x 7 = 70

First, we'll simplify the expression inside the parentheses:
(X/4 + 6) x 7 = 70

Now, we'll divide both sides of the equation by 7 to get rid of the multiplication:
(X/4 + 6) = 70/7
(X/4 + 6) = 10

Subtract 6 from both sides:
(X/4 + 6) - 6 = 10 - 6
X/4 = 4

Multiply both sides by 4:
4(X/4) = 4(4)
X = 16

Therefore, the solution to equation k) is X = 16.

l) 5 x (7 + 3 x X) = 140

First, we'll simplify the expression inside the parentheses:
7 + 3 x X = 7 + 3X

Now, we'll substitute this simplified expression back into the equation:
5 x (7 + 3X) = 140

To isolate the variable X, we'll divide both sides of the equation by 5:
(7 + 3X) = 140/5
7 + 3X = 28

Subtract 7 from both sides:
7 - 7 + 3X = 28 - 7
3X = 21

Divide both sides by 3:
3X/3 = 21/3
X = 7

Therefore, the solution to equation l) is X = 7.