We are given that diameter of base = 8 ft. Therefore, the radius of circular base = 8/2 = 4 ft
In triangle OAB and OCD
OA/AB = OC/CD
⇒ AB = 3×4/12 = 1ft
Therefore, the volume of remaining part = Volume of entire cone - Volume of smaller cone
⇒ 1/3×π×42×12-1/3×π×12×3
⇒ 1/3×π×189
⇒ 22/7×3×189
⇒ 198 cubic ft

In the following figure chord MN and chord RS intersect at point D. If RD = 15, DS = 4, MD = 8, find DN by completing the following activity: 
Activity :
\(\therefore\) MD \(\times\) DN = \(\boxed{\phantom{SD}}\) \(\times\) DS \(\dots\) (Theorem of internal division of chords)
\(\therefore\) \(\boxed{\phantom{8}}\) \(\times\) DN = 15 \(\times\) 4
\(\therefore\) DN = \(\frac{\boxed{\phantom{60}}}{8}\)
\(\therefore\) DN = \(\boxed{\phantom{7.5}}\)