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Rt = R1+R2+R3
Rt = 9 ohms + 3 ohms + 12 ohms = 24 ohms
Note:
It = Et/Rt
It = 12 volts/ 24 ohms = 0.5 amps
To find Rt in the circuit:
Rt = R1+R2
Rt = 6 ohms + 6 ohms = 12 ohms
To find It in the circuit:
It = Et/Rt
It = 12 volts/ 12 ohms = 1 amp
To find Rt in the circuit:
Rt = R1+R2+R3+R4
Rt = 3 ohms + 2 ohms + 6 ohms + 1 ohm = 12 ohms
To find It in the circuit:
It = Et/Rt
It = 12 volts/ 12 ohms = 1 amp
You have to find Rt first:
Rt = R1+R2+R3
Rt = 3 ohms + 1 ohm + 2 ohms = 6 ohms
You have to then find It using Ohm's Law:
It = Et/Rt
It = 12 volts/ 6 ohms = 2 amps
But we know that there is only one path for current flow  therefore the
current flow is the same at any point in the circuit:
It=I1=I2=I3....
A1=A2=A3=A4= 2 amps