The distance between city A and city B is 90 miles. Halfway between city A and city B, there is a city C. Nate started to bike from city A to city B at a rate of 12 mph, while, simultaneously Alex started to bike from city C to city B at a rate of 15 mph. Alex reached city B and turned around immediately at the same rate. At what distance from city B, did Alex and Nate meet each other?

For every corresponding pair of cross sections, the area of the cross section of a sphere with radius r is equal to the area of the cross section of a cylinder with radius and height 2r minus the volume of two cones, each with a radius and height of r. a cross section of the sphere is and a cross section of the cylinder minus the cones, taken parallel to the base of cylinder, is the volume of the cylinder with radius r and height 2r is and the volume of each cone with radius r and height r is 1/3 pie r^3. so the volume of the cylinder minus the two cones is therefore, the volume of the cylinder is 4/3pie r^3 by cavalieri's principle. (fill in options are: r/2- r- 2r- an annulus- a circle -1/3pier^3- 2/3pier^3- 4/3pier^3- 5/3pier^3- 2pier^3- 4pier^3)