Robert has built a mechanical model solar system with three balls representing planets at the end of rods. Each rod is attached
to the center ball, representing the sun. The planets are lined up when he turns on the motor. The innermost planet makes a revolution in 12 seconds, the middle planet makes a revolution in 20 seconds, and the outermost planet makes a revolution in 28 seconds. After how many seconds will the planets be lined up again?
