Two lenses, a convex lens of focal length 20 cm & a concave lens of a focal length 25 cm, are held to close each other. The focal length of the combination of the lenses is:

The power of combination of two lenses is P = P₁ + P₂,
Where P₁ is the power of the convex lens of focal length 20 cm & P₂ is power of concave lens of focal length of 25 cm.
As Power = P = 1/ f (inm)
Therefore, P = 1/f₁ + 1/f₂
= + 100/20 – 100/25 = +1