Consider the following premises. If Robert drinks water, then Joseph is fast. Joseph is not fast. Joseph is fast if and only if Robert is drinking water. Can we infer the following from them? Robert is drinking water.
 From the fact that Joseph is fast if and only if Robert is drinking water we can infer that if Joseph were fast, then Robert is drinking water. From the fact that if Robert drinks water, then Joseph is fast, and that if Joseph were fast, then Robert is drinking water we can infer that Robert is drinking water if and only if Joseph is fast. Finally, from the fact that Robert is drinking water if and only if Joseph is fast, and that Joseph is not fast we can infer that Robert is not drinking water, which contradicts that Robert is drinking water. Therefore, the answer is no.