Geometry - Right circular cone - volume/area

First of all refer this figure http://0.tqn.com/d/math/1/0/t/L/conerr.jpg

From figure its clear that radius and height makes right angle and the slant height can become the hypotenuse. Then by Pythagoras theorem
hypotenuse = square root ( base ^2 + altitude ^2 )
Here slant height = square root ( radius^2 + height ^2)

1) slant ht^2 = r^2+h^2= 144+256=400
slant ht = square root of 400 = 20

OPTION B

2)Cone has a circular base and slanted faces.
Total surface area = slant area+base area = pi*r*s + pi*r^2
Base area = pi*r^2 = pi *16 = 16 pi sq.in(base is circle)

Slant area = pi*r*s = pi* 4* s
s ----Slant ht = square root ( r^2 + h^2 ) = sqrt(16+ 9 ) = sqrt(25) = 5
Slant area = pi*4*5= 20pi sq.in
Total surface area = 20+16 =36 pi sq.in

OPTION C
3) Volume = (1/3)*pi*r^2 *h= 24pi cubic cm
V=(1/3)*PI*9*H = 24PI
Cancelling pi on both sides
1/3 * 9 * h = 24
Cancelling 3 on numerator and 9 on the left hand side gives 3 and that 3 on left hand side and 24 on right hand side gives 8 on right hand side (3*8=24)
3h = 24
h = 24/3 = 8

OPTION B