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Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used is ______ cm.

Asked in JEE Main 24th Jan 2nd Shift 2026 · Mirror formula and magnification

Answer: (2) -15

Step-by-step solution

Real image, m=-3, so v=3u and both lie in front of the mirror.

Separation |v|-|u|=2|u|=40 cm, so u=-20 cm and v=-60 cm.

1/f=1/v+1/u=-1/(60)-1/(20)=-4/(60)

f=-15 cm

Why the other options are wrong

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