Final answer:
The value of register r20 cannot be determined with the given information, which relates to calculating equivalent resistance in a physics problem, and not computer code.
Step-by-step explanation:
The value of register r20 after the execution of the provided code cannot be determined, as the information given pertains to a physics problem regarding equivalent resistance rather than computer code. When calculating equivalent resistance, if the resistors are in series, their resistances are added. If they are in parallel, the reciprocal of the total resistance is equal to the sum of the reciprocals of each resistance. However, since no specific values or configurations are provided, the question seems to be missing the necessary information to solve either the physics problem or the question about the register value in a piece of code.