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to use the Math.Random() class firstly:

import the Random class:
import java.util.Random;

then in your main class section to create a Random object:
Random rand = new Random();

then to...

can you explain the three options: 218, 312, 731. Do you want the program to randomly pick one of these 3 and then add 8 more random digits to create a 11 digit number?