problem: Ohm's law states that $V=IR$ for a simple circuit, where $V$ is the total voltage, $I$ is the amount of current running through the resistor and $R$ is the resistance of the resistor. If $I=40$ amperes when $R=3$ ohms, find the amount of current (in amperes) when the resistor has a resistance of $20$ ohms. Assume voltage remains constant.
solution: If $V=IR$ for a constant voltage $V$, then the total voltage of the circuit can be expressed as $V=(40)(3)=120$. Thus, when the resistance $R$ is $20$ ohms, the equation becomes: \begin{align*} 120& =(I)(20)
  \\\Rightarrow\qquad I& =\frac{120}{20}
  \\ I& =\boxed{6}
  \end{align*}