Answer
10.2 m.
Work Step by Step
Use the lens equation.
$$\frac{1}{s}+\frac{1}{s’}=\frac{1}{f}$$
The image is to be formed on the sensor, so s’=20.4 cm.
Solve for the object distance.
$$s=\frac{s’f}{s’-f}=\frac{(20.4cm)(20.0cm)}{(20.4cm-20.0cm)}=1020 cm$$
The object distance s is much greater than f. As expected, the image is focused just outside the focal point.
