Twelve conductors with a resistance of 15 Ohm each are connected 4 in series in 3 parallel circuits. Determine the total resistance.

When connected in series, the total resistance is:
Rtot1 = R1 + R2 + R3 + R4
Rtot1 = 15 * 4 = 60 (Ohm)
In the second section, series-connected conductors, the total resistance is:
Rtot2 = R1 + R2 + R3 + R4 = 15 * 4 = 60 (Ohm)
On the third, exactly the same: Rtot3 = 60 (Ohm)
Now, the circuit, as it were, consists of three parallel-connected conductors with a resistance R = 60 (Ohm)
To find their total resistance, you need the resistance of one conductor (because their resistance is equal) divided by the number of conductors
Rtot = R / 3
Rtot = 60/3 = 20 (Ohm)