Chapter 35 - Optical Instruments - Exercises and Problems - Page 1017: 14

$s = 4.2~cm$

We can find $f$: $f = \frac{25~cm}{M}$ $f = \frac{25~cm}{5}$ $f = 5.0~cm$ Note that the image is a virtual image so $s' = -25~cm$ We can find $s$: $\frac{1}{s}+\frac{1}{s'} = \frac{1}{f}$ $\frac{1}{s} = \frac{1}{f} - \frac{1}{s'}$ $\frac{1}{s} = \frac{1}{5.0~cm} - \frac{1}{-25~cm}$ $\frac{1}{s} = \frac{6}{25~cm}$ $s = \frac{25~cm}{6}$ $s = 4.2~cm$