Since rays are converging, so it is a converging lens i.e., convex lens. Rays coming from sun are considered to be coming from infinity and a converging lens converges all the incident rays from infinity at focus.Convex lens forms real image of a distant object and its focal length is positive in sign.So, here focal length $=20\ \text{cm}=0.2\ \text{m}$Power of convex lens gives the degree of converging ability of the lens.Power of lens is the reciprocal of focal length. Power $=\;\frac{1}{f}=\;\frac{100}{20}=5\ \text{D}$Therefore, power of convex lens with $20\ \text{cm}$ focal length is $+5\ \text{D}$.