If a current source, EMF of which is 4.5 V, is connected to an external resistance of 8.5 Ohm, then the current in the circuit

If a current source, EMF of which is 4.5 V, is connected to an external resistance of 8.5 Ohm, then the current in the circuit will become equal to 0.5 A. What is the internal resistance of the source?

The current in a closed circuit I depends on the EMF, internal resistance r and external resistance R according to the law:
I = E / (r + R);
I * (r + R) = E;
Ir + IR = E;
r = (E-IR) / I = (4.5 V – 8.5 Ohm * 0.5 A) / 0.5 A = 0.5 Ohm.
Answer: The internal resistance of the source is 0.5 Ohm.