Cone Surface Area?

[Vlatko]

How to calculate the height of a cut cone (if it is called that in english), if the surface area is 320pi cm2, R = 10 and r = 5. See picture for explanation.

First is this correct:

P -surface area
P=B+B1+M

B= 10^2*Pi

B1=5^2*pi

M=(R+r)*s*Pi
M=15*s*Pi

so
P=100*pi + 25*Pi + 15*Pi*s
320 = 100 + 25 + 15*s
195 = 15*s
s = 13

H^2 = s^2 - (R - r)^2
H^2 = 169 - 25
H^2 = 144
H = 12

It seems correct but i have a solution which says it is not. Can the solution be wrong?