Answer
a. About 450 mm.
b. 478 mm.
Work Step by Step
a. See the ray diagram. The tick marks are 100 mm apart. The object distance is about 450 mm.
b. Use equation 23-8 to find the object distance.
$$\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{f}$$
$$d_o=\frac{fd_i}{d_i-f}=\frac{(215mm)(391mm)}{391mm-215mm}=478mm$$
