To extend what Rahul Gupta said:
You can use Java function int random = Random.nextInt(n).
This returns a random int in the range [0, n-1].
I.e., to get the range [20, 80] use:
final int random = new Random().nextInt(61) + 20; // [0, 60] + 20 => [20, 80]
To generalize more:
final int min = 20;
final int max = 80;
final int random = new Random().nextInt((max - min) + 1) + min;