A concave lens of focal length 35 cm is placed in contact with a convex lens of focal length 20 cm. What is the power of the combination?

First, we consider the power of the first lens which is concave. Since it is a concave lens the power will be in negative.
We know that Power(P)=1/Focal length(in metres)=100/Focal length(in centimetre)
Power of lens1 will be = -100/35=    -2.8D
Similarly,power of lens2 will be = 100/20=  +5D

Power of combination=p1+p2
5+(-2.8)=5-2.8=+2.2
Hence,the power of combination will "+2.2D"