What is the internal resistance of a current source with an EMF of 12 V if, when a resistor with an unknown resistance is connected to it, a current of 2 A flows through the circuit? A voltmeter connected to the source terminals shows 8 V.

Explanation:
Second question:
U
=
I
2
×
R
2
=
4
A
×
9
Ω
=
36
V
First question:
You could use
1
R
=
1
R
1
+
1
R
2
+
1
R
3
=
1
18
+
1
9
+
1
6
=
1
3
→
R
=
3
Ω
Or:
You can calculate the currents through the other resistors, add them all up and recalculate the total resistance (voltage already calculated):
I
1
=
U
R
1
=
36
V
18
Ω
=
2
A
I
2
=
4
A
(as given)
I
3
=
U
R
3
=
36
V
6
Ω
=
6
A
Now
R
=
U
I
=
36
V
2
A
+
4
A
+
6
A
=
36
V
12
A
=
3
Ω

My answer is opptionc, just check with anyone else