In parallel, neither resistor knows about the other one, and they have no effect on each other.
Each resistor is connected straight to the battery terminals. If the battery is hefty enough, it supplies current to each resistor as if the other one weren't there.
Current through a resistor = (voltage across it) / (its resistance).
Current = (20 V) / (40 ohms)
Current = 1/2 Ampere.
