Question

what voltage must be supplied by a battery to force 0.4 through a series circuit consisting of a 7 ohm lamp a 10 ohm rheostat, and a 12 ohm soldering iron?

Answer 1

When the resistors in the circuit are in series, you can add them all together to get an equivalent resistance.

So the three resistors in series are 7, 10, and 12 ohms, adding to 29 ohms.

You should know too that Voltage = Current * Resistance

So, you are asked to find the voltage if current = 0.4 amps and resistance = 29 ohms