Ohm's law allows us to find the equivalent resistance of the circuit. In fact:
where
V is the voltage of the battery
I is the current in the circuit
is the equivalent resistance
Re-arranging the equation and using the data of the problem, we get
The three resistors are in series, so their equivalent resistance is just the sum of the three resistances:
Since we know the value of the equivalent resistance,
, we can find the value of R3: