A battery of unknown emf E and internal resistance r is connected in a circuit as shown in the figur 

A battery of unknown emf E and internal resistance r is connected in a circuit as shown in the figure
When the key (K) is open, the voltmeter reads 10·0 V and ammeter reads 0 A. In the closed circuit, the voltmeter reads 6·0 V and ammeter reads 2·0 A. Calculate
(a) emf of the battery,
(b) internal resistance of the battery (r), and
(c) external resistance (R)
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