A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is

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(a) – 30 cm

(b) – 20 cm

(c) – 40 cm

(d) – 60 cm

Answer: (b) – 20 cm

Explanation: In this case, the focal length of the concave mirror is – 20 cm.