Given a soda can with a volume of 21 and a diameter of 6, what is the volume of a cone that fits perfectly inside the soda can?

Volume of a can:
21 = r² π h
21 = 3² π h
21 = 9 π h
h = 21/ 9π = 7/ 3π 
Volume of a cone that fits perfectly inside a can:
V = 1/3 r² π h = 1/3 · 9 π · 7 / 3π = 7
Or you can use: V cone = 1/3 V cylinder ( with the same basis )
V cone = 1/3 * 21 = 7