What is the internal resistance of a 12.0-v car battery whose terminal voltage drops to 8.3 v when the starter motor draws 89 a ?

The relationship between the terminal voltage V of a battery and its electromotive force  is given by

where I is the current and r the internal resistance.
In our problem , and when a current of  flows, the terminal voltage is . So we can find the value of the internal resistance:

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