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(B) For a concave mirror,the magnification $m$ is given by $m = \frac{f}{f - u}$.Since the image is real,the magnification $m$ must be negative. Given

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$15$

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(B) For a concave mirror,the magnification $m$ is given by $m = \frac{f}{f - u}$.Since the image is real,the magnification $m$ must be negative. Given $m = -3$ and the object distance $u = -20 \ cm$.Substituting these values into the formula:$-3 = \frac{f}{f - (-20)}$$-3 = \frac{f}{f + 20}$$-3(f + 20) = f$$-3f - 60 = f$$-4f = 60$$f = -15 \ cm$.The magnitude of the focal length is $15 \ cm$.

$A$ concave mirror gives an image three times as large as the object placed at a distance of $20 \ cm$ from it. For the image to be real,the focal length should be ...... $cm$.

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