Correct Answer: Option C

Explanation

V = \(\frac{U}{2}\) 

f = 20cm

\(\frac{1}{f} = \frac{1}{u} + \frac{1}{v}\) = \(\frac{1}{20} = \frac{1}{u} + \frac{2}{u}\)

\(\frac{1}{20} = \frac{3}{u}\) 

u = 60cm.

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