First weighing as above. Keep the three groups intact. Whether or not the scale balanced, weigh the unweighed group against a group already weighed. Now you know which group has the different ball and whether it is heavier or lighter. Third weighing as above.
First weighing as above. Keep the three groups intact. Whether or not the scale balanced, weigh the unweighed group against a group already weighed. Now you know which group has the different ball and whether it is heavier or lighter. Third weighing as above.