We use ohm's law to solve this problem. Since the incoming current is 4A, we say I=4A. The voltmeter reads a voltage drop of 20V across the resistor. This means that if we apply ohm's law on the resistor, we can say that V=I*R. Therefore, 20=4*R. This gives us that value of R to be 5 ohms.
This was the theoretical ideal case. Now, with non ideal meters, in the real world temperature increases and so the resistance also increases.
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We use ohm's law to solve this problem. Since the incoming current is 4A, we say I=4A. The voltmeter reads a voltage drop of 20V across the resistor. This means that if we apply ohm's law on the resistor, we can say that V=I*R. Therefore, 20=4*R. This gives us that value of R to be 5 ohms.
This was the theoretical ideal case. Now, with non ideal meters, in the real world temperature increases and so the resistance also increases.