On a snorkeling trip,antonia dove at least 7 times as deep as lucy did. if antonia dove 35 feet below the oceans surface,what was the deepest lucy dove?
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Solving for the volume of an oblique cone is the same as solving for the volume of a right circular  cone. 

v =  π r² h/3

since the slant length is given, we need to solve for the height. in a right triangle the slant length is the hypotenuse and the given radius serves as the short leg. we use the pythagorean theorem to solve for the height.

a² + b² = c²
a² + 8² = 17²
a² = 17² - 8² = 289 - 64
a² = 225
a =  √225 = 15

volume of oblique cone=  π * 8² * 15/3 =  π * 64  * 5 = 320π cubic units

volume of cylinder =  π r² h =  π * 8² * 15 =  π * 64 * 15 = 960π cubic units

volume of unfilled portion of the cylinder:  
960π cubic units - 320π cubic units = 640π cubic units.   choice c.