What is a sample voltage problem and formula?

The formula for voltage is: E = I x R

(E is voltage measured in volts, I is current measured in amperes or amps, R is resistance measured in ohms).

Sample problem:

An Mp3 player has 40 ohms of resistance and 0.1 amps of current flowing through it. What is the voltage that supplies the Mp3 player?

Start with the formula, E = I x R.

Put the values given into the formula: E = 0.1 x 40

Now, do the math: E = 4 volts