Ohm's law
For example, a scientist knows that the value of the voltage is 20V. Resistance is known, which is in the light bulb, is 10 Ω.  Now we need to find the other unknown variable, which is current. The Ohm’s law formula can be used to solve it. With the two known variables, V(voltage) and R(resistance), the only variable left to find is I(current).