Consider the following premises. p_20 <-> r_2. ~p_20. r_2 or ~p_20. Can we infer ~r_2 or q from them? If possible, name the inference rules being used at each step.
 From p_20 <-> r_2 we can infer r_2 -> p_20 via biconditional elimination. From r_2 or ~p_20 we can infer p_20 -> r_2 via rewriting. From r_2 -> p_20, p_20 -> r_2 we can infer r_2 <-> p_20 via biconditional introduction. From r_2 <-> p_20, ~p_20 we can infer ~r_2 via biconditional elimination. Finally, from ~r_2 we can infer ~r_2 or q via addition. Therefore, the answer is yes.