Answer
22.0 cm.
Work Step by Step
Apply the lensmaker’s formula.
$$\frac{1}{f}=(n-1)(\frac{1}{R_1}-\frac{1}{R_2})$$
Since f is positive, we choose $R_1=R$ and $R_2=-R$, where R is the magnitude of the radius of curvature.
$$\frac{1}{f}=(n-1)(\frac{1}{R}-\frac{1}{-R})=(n-1) \frac{2}{R}$$
$$R=2(n-1)f=2(1.55-1)(20.0 cm)=22.0 cm$$
