Why Ohm's Law says current equals voltage divided by resistance, I = E/R. Here 200 volts divided by 100 ohms gives 2 amperes. The resistor is connected directly across the source, so the full 200 volts appears across it.
Watch out The 0.5 ampere answer comes from dividing the resistance by the voltage instead of the other way around, and 20,000 amperes comes from multiplying voltage by resistance.
Volts on top: I = E/R. If your answer is bigger than the voltage number, you probably multiplied.