Here, focal length of concave lens, `f_(1) = -25 cm`, focal length of convex lens, `f_(2) = + 20 cm`

power of the combination, `P = ?`, focal length of the combination, `F = ?`

Clearly, `P_(1) = 100/(f_(1)(cm)), P = 100/(-25) = -4` dioptre and `P_(2) = 100/(f_(2)(cm)) = 100/20 = + 5` dioptre

As `P = P_(1) + P_(2), P = - 4 + 5 = 1` dioptre

Thus, `F = 100/P = 100 cm = 1 m`.

Positive sign of P and F indicates that the combination of two given lenses behaves as a convex lens.