Consider the following premises. s -> q_3. s -> q_3. q_3 -> p_15. p_15 <-> s. Can we infer q_3 <-> s from them?
 From p_15 <-> s we can infer p_15 -> s. From q_3 -> p_15, p_15 -> s we can infer q_3 -> s. Finally, from q_3 -> s, s -> q_3 we can infer q_3 <-> s. Therefore, the answer is yes.