$3: 4:: X: 8$

Step 1 ：Given the proportion $3: 4:: X: 8$

Step 2 ：To solve for X, we can cross-multiply and then divide by the remaining number. This will give us the value of X.

Step 3 ：Cross-multiplication gives us $3 \times 8 = 24$

Step 4 ：Dividing 24 by 4 gives us $X = \frac{24}{4}$

Step 5 ：Solving for X gives us $X = 6.0$

Step 6 ：Final Answer: The value of X is \(\boxed{6.0}\)