Question

what is the voltage across a resistor of 25 ohms when current of 200 millimeters is flowing though it (note: 1 ampere = 1000 milliampere

Answer 1

You take the basic formula of E = I x R, solve for R -> R = E / I. In other words, take the required voltage drop (in volts) and divide by the current (in amps) in the resistor and determine the resistance (R) in ohms.