you may use similar triangles there..

suppose that the radius of the cone is R and the radius of the cone at the hight of 9 ft is r, then the ratio is given by

which implies that

given th situation, the volume of the cone when full is

while when if 9 ft full,

then, the percentage is

then multiply by 100%.. Ü

for b)

you are looking for h. so, using similar triangles again, you have

so that

so, 75% full,

so,

can you continue? you will notice that R^2 will cancel and you can now solve for h. Ü