Suppose that a large black hole forms two images of a background source separated by an angle θ. Let the fluxes of the two images be F₊ and F₋ < F₊. Show that the flux from the source would be F₊ − F₋ if there were no lens and that the black hole should be located an angular distance [1+ (F₋/F₊)^−1/2]⁻¹θ along the line from the brighter image to the fainter one.