you didn't upload the picture. But I think you should find the volume of the given groups and also find the volume of the biggest pyramid then divide the volume of the biggest pyramid by the volume of the group
16 cm - 2.5 cm = 13.5 cm, which is 1/4 of the diameter of the large gear. Take 13.5 cm * 4, and you have the diameter of the large gear, which is 54 cm.