Answer
9.0 cm.
Work Step by Step
The plane surface has an infinite radius of curvature. Let this be surface 2.
Use the lensmaker’s equation.
$$\frac{1}{f}=(n-1)(\frac{1}{R_1}+\frac{1}{R_2})$$
$$\frac{1}{f}=(n-1)(\frac{1}{R_1}+\frac{1}{\infty})$$
$$\frac{1}{f}=\frac{ n-1}{R_1}$$
$$R_1=(n-1)f=(1.55-1)(16.3cm)=9.0cm$$