Answer:
(i) The lens B should be used as objective because it has larger focal length and larger radius. (ii) In normal adjustment, the distance between objective and eyepiece \[={{f}_{0}}+{{f}_{e}}=20+5=25cm\] Distance required to be increased between the two lenses \[=25-15=\mathbf{10cm}\] (iii) Magnifying power of the telescope in normal adjustment, \[m=\frac{{{f}_{0}}}{{{f}_{e}}}=\frac{20cm}{5cm}=\mathbf{4}\]
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