Double convex lenses are to be manufactured with both a glass of same refractive index and same radius of curvature.Given \\mu=1.55 and one glass radius be R_1=R while other be R_2=-R Focal length f=20cm Now \\begin{equation}\\frac{1}{f}=(\\mu-1)[\\frac{1}{R_1}-\\frac{1}{R_2}]=(\\mu-1)[\\frac{1}{R}+\\frac{1}{R}]=\\frac{1.1}{R}\\end{equation}So, radius of curvature R=22cm