An object 10 cm high is placed at a distance of 20 cm in front of a convex lens. An erect image 25 cm high is formed on the same side of the lens as the object is. Find the focal length of the lens if the distance between the object and the image formed is 30 cm.

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• A 14.28
• B 33.33
• C -33.33
• D -14.28

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Answer: B. 33.33

Oh = 10 cm u = -20 cm, Ih= 25 cm

m = v/u = Ih/Oh

v/(-20)= 25/10

V = -50 cm

1/f = 1/v - 1/u

1/f = - 1/50 + 1/20 = (-2+5)/100 = 3/100

f = 100/3
=33.33 cm Ans.

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