An object is placed 30 cm in front of a concave mirror of focal length 10 cm. What is the image distance?

Using the mirror formula $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$, with f = 10 cm and u = 30 cm: $\frac{1}{v} = \frac{1}{10} - \frac{1}{30} = \frac{3-1}{30} = \frac{2}{30}$, giving v = 15 cm. The image is real and forms 15 cm in front of the mirror.