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Ohm's law states that $V = I R$ 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.