The number of dry cells, each of $e.m.f.$ $1.5\,volt$ and internal resistance $0.5\, ohm$ that must be joined in series with a resistance of $20\, ohm$ so as to send a current of $0.6\,A$ through the circuit is
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In series $i = \frac{{nE}}{{nr + R}}$ $ \Rightarrow $ $0.6 = \frac{{n \times 1.5}}{{n \times 0.5 \times 20}}$ $ \Rightarrow $ $n = 10$
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