A circuit contains two devices that are connected in parallelIf the resistance of one of these device is 12 ohms and the resistance of the other device is 4 ohms the total resistance of the two divice?

For resistances in parallel, total effective resistance is [ 1 divided by (1/R1 + 1/R2 + 1/R3 + . . . . . etc.) ]

For two resistors, this expression can be massaged to show that the net effective resistance is

(R1 R2) / (R1 + R2).

(12 x 4) / (12 + 4) = 48/16 = 3 ohms.