Voltage is a measure of electric energy per unit charge. Electrical current, the flow of electrons, is powered by voltage and travels throughout a circuit and becomes impeded by resistors, such as light bulbs. Finding the voltage drop across a resistor is a quick and simple process.
Look at the structure of the circuit to identify a parallel circuit. If a resistor is connected to other resistors and battery at both nodes, then you have a parallel circuit.
Find the sum of the reciprocals of all the resistors in the parallel resistor. The total resistance formula can be found at the Electronic-Tutorials website. Check that you accounted for all the resistors in the circuit as there may be other resistors connected in a series to the parallel resistor. If this is the case, add the total resistance of the parallel resistor to all the other resistors connected in series in the circuit.
Multiply the amount of current (which should be given) by the total resistance of the parallel resistor. The voltage drop across a resistor is equal to the product of the amount of current passing through it and its resistance. The voltage drop across a particular resistor in the parallel resistor is equal to the voltage drop across the entire parallel resistor.
All formulas can be found at the Electronics-Tutorials website.