A resistor is connected in series with a 3-ohm resistor and then connected in the original circuit. If the current passing through becomes 4 / 5 of the original current, what is the resistance value of this resistor?

U/R=Io
U/(R+3)=(4/5)Io
The two forms are similar
(U/R)/[U/(R+3)]=[Io/(4Io/5)]
R/(R+3)=4/5
The resistance of this resistor is
R = 12 Ω

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