If the object distance and the image distance from a concave mirror is -20 cm, what is the focal length of the mirror?

Correct option is B

​$$\textbf{For a spherical mirror, the mirror formula is:}\\[8pt]\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u}\\[12pt]\text{where}\\[6pt]\bullet\ u = \text{object distance}\\\bullet\ v = \text{image distance}\\\bullet\ f = \text{focal length}\\[10pt]\text{(All distances are taken according to the sign convention.)}\\[16pt]\textbf{Given}\\[8pt]\bullet\ \text{Object distance: } u = -20~\text{cm}\\\bullet\ \text{Image distance: } v = -20~\text{cm}\\[16pt]\textbf{Calculation}\\[10pt]\dfrac{1}{f}= \dfrac{1}{-20} + \dfrac{1}{-20}= -\dfrac{1}{20} - \dfrac{1}{20}= -\dfrac{1}{10}\\[12pt]f = -10~\text{cm}$$​​