Algorithm of fallout

Let's say there's a 30 percent chance. What is it? algorithms or Methodsto degrade the subject by approximately 30 per cent? More interesting. description of this methodnot the code itself. How real projects and Games Or where can we read about it?

It is known that existing generators (pseudo) of type of emergency numbers https://docs.oracle.com/javase/8/docs/api/java/lang/Math.html#random returns usually within range
[0.0; 1.0)
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Accordingly, for the power generation, a 30 per cent chance should be selected within this range of a 0.3 wide cut:double r = Math.random();
if (r < 0.3) // [0.0, 0.3)
if (0.2 <= r && r < 0.5) // [0.2, 0.5)
if (r >= 0.7) // [0.7; 1.0)