A complete circuit contains two parallel-connected devices ana a generator for providing the electromotive force the resistance of the first device is 12 ohms the resistance of the second device is 4?

According to Ohm's law of resistance in parallel, 1/RT=1/R1+1/R2+1/R3... so 1/RT=1/12+1/4=1/12+3/12=4/12=1/3 and 1/(1/3)=3.

The total resistance of the two devices is 3.

There is a simpler special case formula for two resistances in parallel:

* RTot = (R1R2) / (R1 + R2) In this case this it would give us (12 x 4) / (12 + 4), or 48 / 16 which equals 3 ohms.
3 ohms