At the end of the battery charging with a current I₁ = 3A, a voltmeter attached to it showed a voltage U₁ = 4.25V. At the beginning of the discharge of the same battery current I₂ = 4A, the voltmeter showed a voltage U₂ = 3.9V. The resistance of the voltmeter is very high. Find the EMF and internal resistance r of the battery.

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