Answer
64 cm.
Work Step by Step
Consider equation 23–9 for the magnification $m=\frac{h_i}{h_o}=-\frac{d_i}{d_o}$.
The image is the same size as the object when $d_i=d_o$. Both numbers are positive because the image is real. Now apply the lens equation.
$$\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{f}$$
$$\frac{1}{d_o}+\frac{1}{d_o}=\frac{1}{f}$$
$$d_o=2f=64 cm$$
The object should be placed 64 cm away.