A concave mirror produces an image of an object such that the distance between the object and image is \( 20 \, \text{cm} \). If the magnification of the image is \( '-3' \), then the magnitude of the radius of curvature of the mirror is:
\( 30 \, \text{cm} \)
\( 7.5 \, \text{cm} \)
\( 3.75 \, \text{cm} \)
\( 15 \, \text{cm} \)
