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+### Statement
+
+$13.1.17.$ [Insert the problem statement]
+
+### Solution
+
+Firstly, the magnification of the object is given by:\
+$k = \frac{s'}{s}$ (1)\
+where $s'$ is the image-mirror distance and $s$ is the object-mirror distance, with $k$ = 3.\
+And the relation for these distances is\
+$\frac{1}{s} +\frac{1}{s'} = \frac{1}{f}$ (2)\
+When object is moved a distance $d$ away from the mirror, the magnification changes\
+$m = \frac{s''}{s+d}$ (3)\
+with $m$ = 0.5.\
+And the relation for these distances is\
+$\frac{1}{s+d}+\frac{1}{s''}=\frac{1}{f}$ (4)\
+From (1) and (2), eliminating term $s'$,\
+$\frac{s}{f} = 1+\frac{1}{k}$ (5)\
+From (3) and (4), eliminating term $s''$,
+$\frac{s}{f}=1+\frac{1}{m}-\frac{d}{f}$ (6)\
+Equaling (5) and (6), and separating $f$,\
+$f = \frac{mkd}{k-m} = 48\;\rm{cm}$
+
+
+#### Answer
+
+[Insert a concise answer or boxed result]