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(D) For a concave mirror,the focal length $f = -30 \, cm$. Given that the image is erect (virtual),the magnification $m$ must be positive. Since the i

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$20$

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(D) For a concave mirror,the focal length $f = -30 \, cm$. Given that the image is erect (virtual),the magnification $m$ must be positive. Since the image is three times the size of the object,$m = +3$. The formula for magnification in terms of focal length and object distance $u$ is $m = \frac{f}{f - u}$. Substituting the given values: $3 = \frac{-30}{-30 - u}$ $3(-30 - u) = -30$ $-90 - 3u = -30$ $-3u = 60$ $u = -20 \, cm$. The negative sign indicates that the object is placed $20 \, cm$ in front of the mirror.

$A$ concave mirror has a focal length of $30 \, cm$. If the image formed is erect (virtual),at what distance from the mirror should an object be placed so that the image is three times the size of the object (in $, cm$)?

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