What is the internal resistance of a 12.0-v car battery whose terminal voltage drops to 8.4 v when the starter draws 70 a ?

The relationship betwen terminal voltage and electromotive force of a battery is given by:

where
V is the terminal voltage
is the electromotive force
I is the current
r is the internal resistance of the battery

In our problem, V=8.4 V,  and the current drawn is , so we can find the internal resistance of the battery by re-arranging the previous equation: