\textbf{172-} In a convex mirror, the object distance from the mirror is $75$ centimeters. If the focal length of the mirror is $20$ centimeters, the image length is how many times the object length?
(1) $3$ \hfill (2) $\dfrac{2}{3}$ \hfill (3) $\dfrac{1}{3}$ \hfill (4) $\dfrac{1}{4}$
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