Rt = 3 ohms
Step-by-step explanation:
Let R1 = 4-ohm resistor
R2 = 12-ohm resistor
For 2 resistors connected in parallel, the total resistance Rt is given by
1/Rt = 1/R1 + 1/R2
or
Rt = R1R2/(R1 + R2)
= (4 ohms)(12 ohms)/(4 ohms + 12 ohms)
= 48 ohms^2/16 ohms
= 3 ohms