Sometimes in electronics you have no alternative but to whip out your calculator and do a little maths. The most likely reason is to calculate how much resistance you need for a given situation, how much current a circuit is going to pull or how much voltage will be dropped between two points in a circuit.
You can make all these calculations by using one of the following formulas derived from Ohm's law:
In the above formulas, V represents voltage (in volts, naturally), I represents current (in amperes), and R represents resistance in ohms.
