To find the value of x in the proportion 0.75 : x :: 5 : 8, we can use the concept of cross-multiplication.

The proportion can be written as: 0.75/x = 5/8

To solve for x, we can cross-multiply and then solve the resulting equation:

0.75 * 8 = x * 5

=> 6 = 5x

Dividing both sides of the equation by 5, we get:

6/5 = x

Therefore, x is equal to 6/5 or 1.2.

Answer: x is equal to 1.2 in the given proportion.