To determine the magnifying power of the eyepiece when a cell is observed at 200x under high power objective (HPO), you can use the formula: Total Magnification = Eyepiece Magnification × Objective Magnification. If the HPO magnification is typically 40x, then the eyepiece magnification would be 200x ÷ 40x = 5x. Therefore, the magnifying power of the eyepiece used is 5x.