Find a red, green and black fake rock among five of each colour by asking seven questions; see http://math.stackexchange.com/questions/1716750.

Question1716750.java

import java.util.ArrayList;
import java.util.List;

public class Question1716750 {
	final static int nrocks = 5;
	final static int ncolours = 3;
	final static int ntotal = nrocks * ncolours;
	final static int nquestions = 7;
	final static int ncases = (int) Math.pow (nrocks,ncolours);
	final static int nsubsets = 1 << (ntotal - 1);
	
	public static void main (String [] args) {
		List<Integer> cases = new ArrayList<Integer> ();
		for (int i = 0;i < ncases;i++)
			cases.add (i);
		System.out.println (recurse (cases,nquestions,""));
	}
	
	static boolean recurse (List<Integer> cases,int questionsLeft,String prefix) {
		if (questionsLeft == 2)
			return true;
		System.out.println (prefix + "depth " + questionsLeft + " : " + cases.size ());
		questionsLeft--;
		prefix += "  ";
		for (int subset = 0;subset < nsubsets;subset++) {
			List<Integer> cases1 = new ArrayList<Integer> ();
			List<Integer> cases2 = new ArrayList<Integer> ();
			for (int c : cases)
				(contains (subset,c) ? cases1 : cases2).add (c);
			if (cases1.size () <= 1 << questionsLeft && cases2.size () <= 1 << questionsLeft) {
				System.out.println (prefix + "subset: " + Integer.toBinaryString (subset));
				if (recurse (cases1,questionsLeft,prefix) && recurse (cases2,questionsLeft,prefix))
					return true;
			}
		}
		return false;
	}
	
	static boolean contains (int subset,int c) {
		for (int i = 0;i < ncolours;i++,c = c / nrocks)
			if ((subset & (1 << (i * nrocks + (c % nrocks)))) != 0)
				return true;
		return false;
	}
}