An eye specialist prescribe spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm . The power of this lens combination , in dioptre.

Question

$\begin{array}{1 1} +1.5\;D \\ -1.5\;D \\ +6.67\;D \\ -6.67\;D \end{array} $